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32 results

io.h

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  • cgroup.c 138.67 KiB
    /*
     *  Generic process-grouping system.
     *
     *  Based originally on the cpuset system, extracted by Paul Menage
     *  Copyright (C) 2006 Google, Inc
     *
     *  Notifications support
     *  Copyright (C) 2009 Nokia Corporation
     *  Author: Kirill A. Shutemov
     *
     *  Copyright notices from the original cpuset code:
     *  --------------------------------------------------
     *  Copyright (C) 2003 BULL SA.
     *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
     *
     *  Portions derived from Patrick Mochel's sysfs code.
     *  sysfs is Copyright (c) 2001-3 Patrick Mochel
     *
     *  2003-10-10 Written by Simon Derr.
     *  2003-10-22 Updates by Stephen Hemminger.
     *  2004 May-July Rework by Paul Jackson.
     *  ---------------------------------------------------
     *
     *  This file is subject to the terms and conditions of the GNU General Public
     *  License.  See the file COPYING in the main directory of the Linux
     *  distribution for more details.
     */
    
    #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
    
    #include "cgroup-internal.h"
    
    #include <linux/cred.h>
    #include <linux/errno.h>
    #include <linux/init_task.h>
    #include <linux/kernel.h>
    #include <linux/magic.h>
    #include <linux/mutex.h>
    #include <linux/mount.h>
    #include <linux/pagemap.h>
    #include <linux/proc_fs.h>
    #include <linux/rcupdate.h>
    #include <linux/sched.h>
    #include <linux/sched/task.h>
    #include <linux/slab.h>
    #include <linux/spinlock.h>
    #include <linux/percpu-rwsem.h>
    #include <linux/string.h>
    #include <linux/hashtable.h>
    #include <linux/idr.h>
    #include <linux/kthread.h>
    #include <linux/atomic.h>
    #include <linux/cpuset.h>
    #include <linux/proc_ns.h>
    #include <linux/nsproxy.h>
    #include <linux/file.h>
    #include <net/sock.h>
    
    #define CREATE_TRACE_POINTS
    #include <trace/events/cgroup.h>
    
    #define CGROUP_FILE_NAME_MAX		(MAX_CGROUP_TYPE_NAMELEN +	\
    					 MAX_CFTYPE_NAME + 2)
    
    /*
     * cgroup_mutex is the master lock.  Any modification to cgroup or its
     * hierarchy must be performed while holding it.
     *
     * css_set_lock protects task->cgroups pointer, the list of css_set
     * objects, and the chain of tasks off each css_set.
     *
     * These locks are exported if CONFIG_PROVE_RCU so that accessors in
     * cgroup.h can use them for lockdep annotations.
     */
    DEFINE_MUTEX(cgroup_mutex);
    DEFINE_SPINLOCK(css_set_lock);
    
    #ifdef CONFIG_PROVE_RCU
    EXPORT_SYMBOL_GPL(cgroup_mutex);
    EXPORT_SYMBOL_GPL(css_set_lock);
    #endif
    
    /*
     * Protects cgroup_idr and css_idr so that IDs can be released without
     * grabbing cgroup_mutex.
     */
    static DEFINE_SPINLOCK(cgroup_idr_lock);
    
    /*
     * Protects cgroup_file->kn for !self csses.  It synchronizes notifications
     * against file removal/re-creation across css hiding.
     */
    static DEFINE_SPINLOCK(cgroup_file_kn_lock);
    
    struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
    
    #define cgroup_assert_mutex_or_rcu_locked()				\
    	RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&			\
    			   !lockdep_is_held(&cgroup_mutex),		\
    			   "cgroup_mutex or RCU read lock required");
    
    /*
     * cgroup destruction makes heavy use of work items and there can be a lot
     * of concurrent destructions.  Use a separate workqueue so that cgroup
     * destruction work items don't end up filling up max_active of system_wq
     * which may lead to deadlock.
     */
    static struct workqueue_struct *cgroup_destroy_wq;
    
    /* generate an array of cgroup subsystem pointers */
    #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys,
    struct cgroup_subsys *cgroup_subsys[] = {
    #include <linux/cgroup_subsys.h>
    };
    #undef SUBSYS
    
    /* array of cgroup subsystem names */
    #define SUBSYS(_x) [_x ## _cgrp_id] = #_x,
    static const char *cgroup_subsys_name[] = {
    #include <linux/cgroup_subsys.h>
    };
    #undef SUBSYS
    
    /* array of static_keys for cgroup_subsys_enabled() and cgroup_subsys_on_dfl() */
    #define SUBSYS(_x)								\
    	DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_enabled_key);			\
    	DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_on_dfl_key);			\
    	EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_enabled_key);			\
    	EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_on_dfl_key);
    #include <linux/cgroup_subsys.h>
    #undef SUBSYS
    
    #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_enabled_key,
    static struct static_key_true *cgroup_subsys_enabled_key[] = {
    #include <linux/cgroup_subsys.h>
    };
    #undef SUBSYS
    
    #define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_on_dfl_key,
    static struct static_key_true *cgroup_subsys_on_dfl_key[] = {
    #include <linux/cgroup_subsys.h>
    };
    #undef SUBSYS
    
    /*
     * The default hierarchy, reserved for the subsystems that are otherwise
     * unattached - it never has more than a single cgroup, and all tasks are
     * part of that cgroup.
     */
    struct cgroup_root cgrp_dfl_root;
    EXPORT_SYMBOL_GPL(cgrp_dfl_root);
    
    /*
     * The default hierarchy always exists but is hidden until mounted for the
     * first time.  This is for backward compatibility.
     */
    static bool cgrp_dfl_visible;
    
    /* some controllers are not supported in the default hierarchy */
    static u16 cgrp_dfl_inhibit_ss_mask;
    
    /* some controllers are implicitly enabled on the default hierarchy */
    static u16 cgrp_dfl_implicit_ss_mask;
    
    /* The list of hierarchy roots */
    LIST_HEAD(cgroup_roots);
    static int cgroup_root_count;
    
    /* hierarchy ID allocation and mapping, protected by cgroup_mutex */
    static DEFINE_IDR(cgroup_hierarchy_idr);
    
    /*
     * Assign a monotonically increasing serial number to csses.  It guarantees
     * cgroups with bigger numbers are newer than those with smaller numbers.
     * Also, as csses are always appended to the parent's ->children list, it
     * guarantees that sibling csses are always sorted in the ascending serial
     * number order on the list.  Protected by cgroup_mutex.
     */
    static u64 css_serial_nr_next = 1;
    
    /*
     * These bitmasks identify subsystems with specific features to avoid
     * having to do iterative checks repeatedly.
     */
    static u16 have_fork_callback __read_mostly;
    static u16 have_exit_callback __read_mostly;
    static u16 have_free_callback __read_mostly;
    static u16 have_canfork_callback __read_mostly;
    
    /* cgroup namespace for init task */
    struct cgroup_namespace init_cgroup_ns = {
    	.count		= REFCOUNT_INIT(2),
    	.user_ns	= &init_user_ns,
    	.ns.ops		= &cgroupns_operations,
    	.ns.inum	= PROC_CGROUP_INIT_INO,
    	.root_cset	= &init_css_set,
    };
    
    static struct file_system_type cgroup2_fs_type;
    static struct cftype cgroup_base_files[];
    
    static int cgroup_apply_control(struct cgroup *cgrp);
    static void cgroup_finalize_control(struct cgroup *cgrp, int ret);
    static void css_task_iter_advance(struct css_task_iter *it);
    static int cgroup_destroy_locked(struct cgroup *cgrp);
    static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
    					      struct cgroup_subsys *ss);
    static void css_release(struct percpu_ref *ref);
    static void kill_css(struct cgroup_subsys_state *css);
    static int cgroup_addrm_files(struct cgroup_subsys_state *css,
    			      struct cgroup *cgrp, struct cftype cfts[],
    			      bool is_add);
    
    /**
     * cgroup_ssid_enabled - cgroup subsys enabled test by subsys ID
     * @ssid: subsys ID of interest
     *
     * cgroup_subsys_enabled() can only be used with literal subsys names which
     * is fine for individual subsystems but unsuitable for cgroup core.  This
     * is slower static_key_enabled() based test indexed by @ssid.
     */
    bool cgroup_ssid_enabled(int ssid)
    {
    	if (CGROUP_SUBSYS_COUNT == 0)
    		return false;
    
    	return static_key_enabled(cgroup_subsys_enabled_key[ssid]);
    }
    
    /**
     * cgroup_on_dfl - test whether a cgroup is on the default hierarchy
     * @cgrp: the cgroup of interest
     *
     * The default hierarchy is the v2 interface of cgroup and this function
     * can be used to test whether a cgroup is on the default hierarchy for
     * cases where a subsystem should behave differnetly depending on the
     * interface version.
     *
     * The set of behaviors which change on the default hierarchy are still
     * being determined and the mount option is prefixed with __DEVEL__.
     *
     * List of changed behaviors:
     *
     * - Mount options "noprefix", "xattr", "clone_children", "release_agent"
     *   and "name" are disallowed.
     *
     * - When mounting an existing superblock, mount options should match.
     *
     * - Remount is disallowed.
     *
     * - rename(2) is disallowed.
     *
     * - "tasks" is removed.  Everything should be at process granularity.  Use
     *   "cgroup.procs" instead.
     *
     * - "cgroup.procs" is not sorted.  pids will be unique unless they got
     *   recycled inbetween reads.
     *
     * - "release_agent" and "notify_on_release" are removed.  Replacement
     *   notification mechanism will be implemented.
     *
     * - "cgroup.clone_children" is removed.
     *
     * - "cgroup.subtree_populated" is available.  Its value is 0 if the cgroup
     *   and its descendants contain no task; otherwise, 1.  The file also
     *   generates kernfs notification which can be monitored through poll and
     *   [di]notify when the value of the file changes.
     *
     * - cpuset: tasks will be kept in empty cpusets when hotplug happens and
     *   take masks of ancestors with non-empty cpus/mems, instead of being
     *   moved to an ancestor.
     *
     * - cpuset: a task can be moved into an empty cpuset, and again it takes
     *   masks of ancestors.
     *
     * - memcg: use_hierarchy is on by default and the cgroup file for the flag
     *   is not created.
     *
     * - blkcg: blk-throttle becomes properly hierarchical.
     *
     * - debug: disallowed on the default hierarchy.
     */
    bool cgroup_on_dfl(const struct cgroup *cgrp)
    {
    	return cgrp->root == &cgrp_dfl_root;
    }
    
    /* IDR wrappers which synchronize using cgroup_idr_lock */
    static int cgroup_idr_alloc(struct idr *idr, void *ptr, int start, int end,
    			    gfp_t gfp_mask)
    {
    	int ret;
    
    	idr_preload(gfp_mask);
    	spin_lock_bh(&cgroup_idr_lock);
    	ret = idr_alloc(idr, ptr, start, end, gfp_mask & ~__GFP_DIRECT_RECLAIM);
    	spin_unlock_bh(&cgroup_idr_lock);
    	idr_preload_end();
    	return ret;
    }
    
    static void *cgroup_idr_replace(struct idr *idr, void *ptr, int id)
    {
    	void *ret;
    
    	spin_lock_bh(&cgroup_idr_lock);
    	ret = idr_replace(idr, ptr, id);
    	spin_unlock_bh(&cgroup_idr_lock);
    	return ret;
    }
    
    static void cgroup_idr_remove(struct idr *idr, int id)
    {
    	spin_lock_bh(&cgroup_idr_lock);
    	idr_remove(idr, id);
    	spin_unlock_bh(&cgroup_idr_lock);
    }
    
    static struct cgroup *cgroup_parent(struct cgroup *cgrp)
    {
    	struct cgroup_subsys_state *parent_css = cgrp->self.parent;
    
    	if (parent_css)
    		return container_of(parent_css, struct cgroup, self);
    	return NULL;
    }
    
    /* subsystems visibly enabled on a cgroup */
    static u16 cgroup_control(struct cgroup *cgrp)
    {
    	struct cgroup *parent = cgroup_parent(cgrp);
    	u16 root_ss_mask = cgrp->root->subsys_mask;
    
    	if (parent)
    		return parent->subtree_control;
    
    	if (cgroup_on_dfl(cgrp))
    		root_ss_mask &= ~(cgrp_dfl_inhibit_ss_mask |
    				  cgrp_dfl_implicit_ss_mask);
    	return root_ss_mask;
    }
    
    /* subsystems enabled on a cgroup */
    static u16 cgroup_ss_mask(struct cgroup *cgrp)
    {
    	struct cgroup *parent = cgroup_parent(cgrp);
    
    	if (parent)
    		return parent->subtree_ss_mask;
    
    	return cgrp->root->subsys_mask;
    }
    
    /**
     * cgroup_css - obtain a cgroup's css for the specified subsystem
     * @cgrp: the cgroup of interest
     * @ss: the subsystem of interest (%NULL returns @cgrp->self)
     *
     * Return @cgrp's css (cgroup_subsys_state) associated with @ss.  This
     * function must be called either under cgroup_mutex or rcu_read_lock() and
     * the caller is responsible for pinning the returned css if it wants to
     * keep accessing it outside the said locks.  This function may return
     * %NULL if @cgrp doesn't have @subsys_id enabled.
     */
    static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
    					      struct cgroup_subsys *ss)
    {
    	if (ss)
    		return rcu_dereference_check(cgrp->subsys[ss->id],
    					lockdep_is_held(&cgroup_mutex));
    	else
    		return &cgrp->self;
    }
    
    /**
     * cgroup_e_css - obtain a cgroup's effective css for the specified subsystem
     * @cgrp: the cgroup of interest
     * @ss: the subsystem of interest (%NULL returns @cgrp->self)
     *
     * Similar to cgroup_css() but returns the effective css, which is defined
     * as the matching css of the nearest ancestor including self which has @ss
     * enabled.  If @ss is associated with the hierarchy @cgrp is on, this
     * function is guaranteed to return non-NULL css.
     */
    static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
    						struct cgroup_subsys *ss)
    {
    	lockdep_assert_held(&cgroup_mutex);
    
    	if (!ss)
    		return &cgrp->self;
    
    	/*
    	 * This function is used while updating css associations and thus
    	 * can't test the csses directly.  Test ss_mask.
    	 */
    	while (!(cgroup_ss_mask(cgrp) & (1 << ss->id))) {
    		cgrp = cgroup_parent(cgrp);
    		if (!cgrp)
    			return NULL;
    	}
    
    	return cgroup_css(cgrp, ss);
    }
    
    /**
     * cgroup_get_e_css - get a cgroup's effective css for the specified subsystem
     * @cgrp: the cgroup of interest
     * @ss: the subsystem of interest
     *
     * Find and get the effective css of @cgrp for @ss.  The effective css is
     * defined as the matching css of the nearest ancestor including self which
     * has @ss enabled.  If @ss is not mounted on the hierarchy @cgrp is on,
     * the root css is returned, so this function always returns a valid css.
     * The returned css must be put using css_put().
     */
    struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgrp,
    					     struct cgroup_subsys *ss)
    {
    	struct cgroup_subsys_state *css;
    
    	rcu_read_lock();
    
    	do {
    		css = cgroup_css(cgrp, ss);
    
    		if (css && css_tryget_online(css))
    			goto out_unlock;
    		cgrp = cgroup_parent(cgrp);
    	} while (cgrp);
    
    	css = init_css_set.subsys[ss->id];
    	css_get(css);
    out_unlock:
    	rcu_read_unlock();
    	return css;
    }
    
    static void __maybe_unused cgroup_get(struct cgroup *cgrp)
    {
    	css_get(&cgrp->self);
    }
    
    static void cgroup_get_live(struct cgroup *cgrp)
    {
    	WARN_ON_ONCE(cgroup_is_dead(cgrp));
    	css_get(&cgrp->self);
    }
    
    static bool cgroup_tryget(struct cgroup *cgrp)
    {
    	return css_tryget(&cgrp->self);
    }
    
    struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
    {
    	struct cgroup *cgrp = of->kn->parent->priv;
    	struct cftype *cft = of_cft(of);
    
    	/*
    	 * This is open and unprotected implementation of cgroup_css().
    	 * seq_css() is only called from a kernfs file operation which has
    	 * an active reference on the file.  Because all the subsystem
    	 * files are drained before a css is disassociated with a cgroup,
    	 * the matching css from the cgroup's subsys table is guaranteed to
    	 * be and stay valid until the enclosing operation is complete.
    	 */
    	if (cft->ss)
    		return rcu_dereference_raw(cgrp->subsys[cft->ss->id]);
    	else
    		return &cgrp->self;
    }
    EXPORT_SYMBOL_GPL(of_css);
    
    /**
     * for_each_css - iterate all css's of a cgroup
     * @css: the iteration cursor
     * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
     * @cgrp: the target cgroup to iterate css's of
     *
     * Should be called under cgroup_[tree_]mutex.
     */
    #define for_each_css(css, ssid, cgrp)					\
    	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)	\
    		if (!((css) = rcu_dereference_check(			\
    				(cgrp)->subsys[(ssid)],			\
    				lockdep_is_held(&cgroup_mutex)))) { }	\
    		else
    
    /**
     * for_each_e_css - iterate all effective css's of a cgroup
     * @css: the iteration cursor
     * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
     * @cgrp: the target cgroup to iterate css's of
     *
     * Should be called under cgroup_[tree_]mutex.
     */
    #define for_each_e_css(css, ssid, cgrp)					\
    	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)	\
    		if (!((css) = cgroup_e_css(cgrp, cgroup_subsys[(ssid)]))) \
    			;						\
    		else
    
    /**
     * do_each_subsys_mask - filter for_each_subsys with a bitmask
     * @ss: the iteration cursor
     * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
     * @ss_mask: the bitmask
     *
     * The block will only run for cases where the ssid-th bit (1 << ssid) of
     * @ss_mask is set.
     */
    #define do_each_subsys_mask(ss, ssid, ss_mask) do {			\
    	unsigned long __ss_mask = (ss_mask);				\
    	if (!CGROUP_SUBSYS_COUNT) { /* to avoid spurious gcc warning */	\
    		(ssid) = 0;						\
    		break;							\
    	}								\
    	for_each_set_bit(ssid, &__ss_mask, CGROUP_SUBSYS_COUNT) {	\
    		(ss) = cgroup_subsys[ssid];				\
    		{
    
    #define while_each_subsys_mask()					\
    		}							\
    	}								\
    } while (false)
    
    /* iterate over child cgrps, lock should be held throughout iteration */
    #define cgroup_for_each_live_child(child, cgrp)				\
    	list_for_each_entry((child), &(cgrp)->self.children, self.sibling) \
    		if (({ lockdep_assert_held(&cgroup_mutex);		\
    		       cgroup_is_dead(child); }))			\
    			;						\
    		else
    
    /* walk live descendants in preorder */
    #define cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp)		\
    	css_for_each_descendant_pre((d_css), cgroup_css((cgrp), NULL))	\
    		if (({ lockdep_assert_held(&cgroup_mutex);		\
    		       (dsct) = (d_css)->cgroup;			\
    		       cgroup_is_dead(dsct); }))			\
    			;						\
    		else
    
    /* walk live descendants in postorder */
    #define cgroup_for_each_live_descendant_post(dsct, d_css, cgrp)		\
    	css_for_each_descendant_post((d_css), cgroup_css((cgrp), NULL))	\
    		if (({ lockdep_assert_held(&cgroup_mutex);		\
    		       (dsct) = (d_css)->cgroup;			\
    		       cgroup_is_dead(dsct); }))			\
    			;						\
    		else
    
    /*
     * The default css_set - used by init and its children prior to any
     * hierarchies being mounted. It contains a pointer to the root state
     * for each subsystem. Also used to anchor the list of css_sets. Not
     * reference-counted, to improve performance when child cgroups
     * haven't been created.
     */
    struct css_set init_css_set = {
    	.refcount		= REFCOUNT_INIT(1),
    	.tasks			= LIST_HEAD_INIT(init_css_set.tasks),
    	.mg_tasks		= LIST_HEAD_INIT(init_css_set.mg_tasks),
    	.task_iters		= LIST_HEAD_INIT(init_css_set.task_iters),
    	.cgrp_links		= LIST_HEAD_INIT(init_css_set.cgrp_links),
    	.mg_preload_node	= LIST_HEAD_INIT(init_css_set.mg_preload_node),
    	.mg_node		= LIST_HEAD_INIT(init_css_set.mg_node),
    };
    
    static int css_set_count	= 1;	/* 1 for init_css_set */
    
    /**
     * css_set_populated - does a css_set contain any tasks?
     * @cset: target css_set
     *
     * css_set_populated() should be the same as !!cset->nr_tasks at steady
     * state. However, css_set_populated() can be called while a task is being
     * added to or removed from the linked list before the nr_tasks is
     * properly updated. Hence, we can't just look at ->nr_tasks here.
     */
    static bool css_set_populated(struct css_set *cset)
    {
    	lockdep_assert_held(&css_set_lock);
    
    	return !list_empty(&cset->tasks) || !list_empty(&cset->mg_tasks);
    }
    
    /**
     * cgroup_update_populated - updated populated count of a cgroup
     * @cgrp: the target cgroup
     * @populated: inc or dec populated count
     *
     * One of the css_sets associated with @cgrp is either getting its first
     * task or losing the last.  Update @cgrp->populated_cnt accordingly.  The
     * count is propagated towards root so that a given cgroup's populated_cnt
     * is zero iff the cgroup and all its descendants don't contain any tasks.
     *
     * @cgrp's interface file "cgroup.populated" is zero if
     * @cgrp->populated_cnt is zero and 1 otherwise.  When @cgrp->populated_cnt
     * changes from or to zero, userland is notified that the content of the
     * interface file has changed.  This can be used to detect when @cgrp and
     * its descendants become populated or empty.
     */
    static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
    {
    	lockdep_assert_held(&css_set_lock);
    
    	do {
    		bool trigger;
    
    		if (populated)
    			trigger = !cgrp->populated_cnt++;
    		else
    			trigger = !--cgrp->populated_cnt;
    
    		if (!trigger)
    			break;
    
    		cgroup1_check_for_release(cgrp);
    		cgroup_file_notify(&cgrp->events_file);
    
    		cgrp = cgroup_parent(cgrp);
    	} while (cgrp);
    }
    
    /**
     * css_set_update_populated - update populated state of a css_set
     * @cset: target css_set
     * @populated: whether @cset is populated or depopulated
     *
     * @cset is either getting the first task or losing the last.  Update the
     * ->populated_cnt of all associated cgroups accordingly.
     */
    static void css_set_update_populated(struct css_set *cset, bool populated)
    {
    	struct cgrp_cset_link *link;
    
    	lockdep_assert_held(&css_set_lock);
    
    	list_for_each_entry(link, &cset->cgrp_links, cgrp_link)
    		cgroup_update_populated(link->cgrp, populated);
    }
    
    /**
     * css_set_move_task - move a task from one css_set to another
     * @task: task being moved
     * @from_cset: css_set @task currently belongs to (may be NULL)
     * @to_cset: new css_set @task is being moved to (may be NULL)
     * @use_mg_tasks: move to @to_cset->mg_tasks instead of ->tasks
     *
     * Move @task from @from_cset to @to_cset.  If @task didn't belong to any
     * css_set, @from_cset can be NULL.  If @task is being disassociated
     * instead of moved, @to_cset can be NULL.
     *
     * This function automatically handles populated_cnt updates and
     * css_task_iter adjustments but the caller is responsible for managing
     * @from_cset and @to_cset's reference counts.
     */
    static void css_set_move_task(struct task_struct *task,
    			      struct css_set *from_cset, struct css_set *to_cset,
    			      bool use_mg_tasks)
    {
    	lockdep_assert_held(&css_set_lock);
    
    	if (to_cset && !css_set_populated(to_cset))
    		css_set_update_populated(to_cset, true);
    
    	if (from_cset) {
    		struct css_task_iter *it, *pos;
    
    		WARN_ON_ONCE(list_empty(&task->cg_list));
    
    		/*
    		 * @task is leaving, advance task iterators which are
    		 * pointing to it so that they can resume at the next
    		 * position.  Advancing an iterator might remove it from
    		 * the list, use safe walk.  See css_task_iter_advance*()
    		 * for details.
    		 */
    		list_for_each_entry_safe(it, pos, &from_cset->task_iters,
    					 iters_node)
    			if (it->task_pos == &task->cg_list)
    				css_task_iter_advance(it);
    
    		list_del_init(&task->cg_list);
    		if (!css_set_populated(from_cset))
    			css_set_update_populated(from_cset, false);
    	} else {
    		WARN_ON_ONCE(!list_empty(&task->cg_list));
    	}
    
    	if (to_cset) {
    		/*
    		 * We are synchronized through cgroup_threadgroup_rwsem
    		 * against PF_EXITING setting such that we can't race
    		 * against cgroup_exit() changing the css_set to
    		 * init_css_set and dropping the old one.
    		 */
    		WARN_ON_ONCE(task->flags & PF_EXITING);
    
    		rcu_assign_pointer(task->cgroups, to_cset);
    		list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :
    							     &to_cset->tasks);
    	}
    }
    
    /*
     * hash table for cgroup groups. This improves the performance to find
     * an existing css_set. This hash doesn't (currently) take into
     * account cgroups in empty hierarchies.
     */
    #define CSS_SET_HASH_BITS	7
    static DEFINE_HASHTABLE(css_set_table, CSS_SET_HASH_BITS);
    
    static unsigned long css_set_hash(struct cgroup_subsys_state *css[])
    {
    	unsigned long key = 0UL;
    	struct cgroup_subsys *ss;
    	int i;
    
    	for_each_subsys(ss, i)
    		key += (unsigned long)css[i];
    	key = (key >> 16) ^ key;
    
    	return key;
    }
    
    void put_css_set_locked(struct css_set *cset)
    {
    	struct cgrp_cset_link *link, *tmp_link;
    	struct cgroup_subsys *ss;
    	int ssid;
    
    	lockdep_assert_held(&css_set_lock);
    
    	if (!refcount_dec_and_test(&cset->refcount))
    		return;
    
    	/* This css_set is dead. unlink it and release cgroup and css refs */
    	for_each_subsys(ss, ssid) {
    		list_del(&cset->e_cset_node[ssid]);
    		css_put(cset->subsys[ssid]);
    	}
    	hash_del(&cset->hlist);
    	css_set_count--;
    
    	list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link) {
    		list_del(&link->cset_link);
    		list_del(&link->cgrp_link);
    		if (cgroup_parent(link->cgrp))
    			cgroup_put(link->cgrp);
    		kfree(link);
    	}
    
    	kfree_rcu(cset, rcu_head);
    }
    
    /**
     * compare_css_sets - helper function for find_existing_css_set().
     * @cset: candidate css_set being tested
     * @old_cset: existing css_set for a task
     * @new_cgrp: cgroup that's being entered by the task
     * @template: desired set of css pointers in css_set (pre-calculated)
     *
     * Returns true if "cset" matches "old_cset" except for the hierarchy
     * which "new_cgrp" belongs to, for which it should match "new_cgrp".
     */
    static bool compare_css_sets(struct css_set *cset,
    			     struct css_set *old_cset,
    			     struct cgroup *new_cgrp,
    			     struct cgroup_subsys_state *template[])
    {
    	struct list_head *l1, *l2;
    
    	/*
    	 * On the default hierarchy, there can be csets which are
    	 * associated with the same set of cgroups but different csses.
    	 * Let's first ensure that csses match.
    	 */
    	if (memcmp(template, cset->subsys, sizeof(cset->subsys)))
    		return false;
    
    	/*
    	 * Compare cgroup pointers in order to distinguish between
    	 * different cgroups in hierarchies.  As different cgroups may
    	 * share the same effective css, this comparison is always
    	 * necessary.
    	 */
    	l1 = &cset->cgrp_links;
    	l2 = &old_cset->cgrp_links;
    	while (1) {
    		struct cgrp_cset_link *link1, *link2;
    		struct cgroup *cgrp1, *cgrp2;
    
    		l1 = l1->next;
    		l2 = l2->next;
    		/* See if we reached the end - both lists are equal length. */
    		if (l1 == &cset->cgrp_links) {
    			BUG_ON(l2 != &old_cset->cgrp_links);
    			break;
    		} else {
    			BUG_ON(l2 == &old_cset->cgrp_links);
    		}
    		/* Locate the cgroups associated with these links. */
    		link1 = list_entry(l1, struct cgrp_cset_link, cgrp_link);
    		link2 = list_entry(l2, struct cgrp_cset_link, cgrp_link);
    		cgrp1 = link1->cgrp;
    		cgrp2 = link2->cgrp;
    		/* Hierarchies should be linked in the same order. */
    		BUG_ON(cgrp1->root != cgrp2->root);
    
    		/*
    		 * If this hierarchy is the hierarchy of the cgroup
    		 * that's changing, then we need to check that this
    		 * css_set points to the new cgroup; if it's any other
    		 * hierarchy, then this css_set should point to the
    		 * same cgroup as the old css_set.
    		 */
    		if (cgrp1->root == new_cgrp->root) {
    			if (cgrp1 != new_cgrp)
    				return false;
    		} else {
    			if (cgrp1 != cgrp2)
    				return false;
    		}
    	}
    	return true;
    }
    
    /**
     * find_existing_css_set - init css array and find the matching css_set
     * @old_cset: the css_set that we're using before the cgroup transition
     * @cgrp: the cgroup that we're moving into
     * @template: out param for the new set of csses, should be clear on entry
     */
    static struct css_set *find_existing_css_set(struct css_set *old_cset,
    					struct cgroup *cgrp,
    					struct cgroup_subsys_state *template[])
    {
    	struct cgroup_root *root = cgrp->root;
    	struct cgroup_subsys *ss;
    	struct css_set *cset;
    	unsigned long key;
    	int i;
    
    	/*
    	 * Build the set of subsystem state objects that we want to see in the
    	 * new css_set. while subsystems can change globally, the entries here
    	 * won't change, so no need for locking.
    	 */
    	for_each_subsys(ss, i) {
    		if (root->subsys_mask & (1UL << i)) {
    			/*
    			 * @ss is in this hierarchy, so we want the
    			 * effective css from @cgrp.
    			 */
    			template[i] = cgroup_e_css(cgrp, ss);
    		} else {
    			/*
    			 * @ss is not in this hierarchy, so we don't want
    			 * to change the css.
    			 */
    			template[i] = old_cset->subsys[i];
    		}
    	}
    
    	key = css_set_hash(template);
    	hash_for_each_possible(css_set_table, cset, hlist, key) {
    		if (!compare_css_sets(cset, old_cset, cgrp, template))
    			continue;
    
    		/* This css_set matches what we need */
    		return cset;
    	}
    
    	/* No existing cgroup group matched */
    	return NULL;
    }
    
    static void free_cgrp_cset_links(struct list_head *links_to_free)
    {
    	struct cgrp_cset_link *link, *tmp_link;
    
    	list_for_each_entry_safe(link, tmp_link, links_to_free, cset_link) {
    		list_del(&link->cset_link);
    		kfree(link);
    	}
    }
    
    /**
     * allocate_cgrp_cset_links - allocate cgrp_cset_links
     * @count: the number of links to allocate
     * @tmp_links: list_head the allocated links are put on
     *
     * Allocate @count cgrp_cset_link structures and chain them on @tmp_links
     * through ->cset_link.  Returns 0 on success or -errno.
     */
    static int allocate_cgrp_cset_links(int count, struct list_head *tmp_links)
    {
    	struct cgrp_cset_link *link;
    	int i;
    
    	INIT_LIST_HEAD(tmp_links);
    
    	for (i = 0; i < count; i++) {
    		link = kzalloc(sizeof(*link), GFP_KERNEL);
    		if (!link) {
    			free_cgrp_cset_links(tmp_links);
    			return -ENOMEM;
    		}
    		list_add(&link->cset_link, tmp_links);
    	}
    	return 0;
    }
    
    /**
     * link_css_set - a helper function to link a css_set to a cgroup
     * @tmp_links: cgrp_cset_link objects allocated by allocate_cgrp_cset_links()
     * @cset: the css_set to be linked
     * @cgrp: the destination cgroup
     */
    static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
    			 struct cgroup *cgrp)
    {
    	struct cgrp_cset_link *link;
    
    	BUG_ON(list_empty(tmp_links));
    
    	if (cgroup_on_dfl(cgrp))
    		cset->dfl_cgrp = cgrp;
    
    	link = list_first_entry(tmp_links, struct cgrp_cset_link, cset_link);
    	link->cset = cset;
    	link->cgrp = cgrp;
    
    	/*
    	 * Always add links to the tail of the lists so that the lists are
    	 * in choronological order.
    	 */
    	list_move_tail(&link->cset_link, &cgrp->cset_links);
    	list_add_tail(&link->cgrp_link, &cset->cgrp_links);
    
    	if (cgroup_parent(cgrp))
    		cgroup_get_live(cgrp);
    }
    
    /**
     * find_css_set - return a new css_set with one cgroup updated
     * @old_cset: the baseline css_set
     * @cgrp: the cgroup to be updated
     *
     * Return a new css_set that's equivalent to @old_cset, but with @cgrp
     * substituted into the appropriate hierarchy.
     */
    static struct css_set *find_css_set(struct css_set *old_cset,
    				    struct cgroup *cgrp)
    {
    	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT] = { };
    	struct css_set *cset;
    	struct list_head tmp_links;
    	struct cgrp_cset_link *link;
    	struct cgroup_subsys *ss;
    	unsigned long key;
    	int ssid;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	/* First see if we already have a cgroup group that matches
    	 * the desired set */
    	spin_lock_irq(&css_set_lock);
    	cset = find_existing_css_set(old_cset, cgrp, template);
    	if (cset)
    		get_css_set(cset);
    	spin_unlock_irq(&css_set_lock);
    
    	if (cset)
    		return cset;
    
    	cset = kzalloc(sizeof(*cset), GFP_KERNEL);
    	if (!cset)
    		return NULL;
    
    	/* Allocate all the cgrp_cset_link objects that we'll need */
    	if (allocate_cgrp_cset_links(cgroup_root_count, &tmp_links) < 0) {
    		kfree(cset);
    		return NULL;
    	}
    
    	refcount_set(&cset->refcount, 1);
    	INIT_LIST_HEAD(&cset->tasks);
    	INIT_LIST_HEAD(&cset->mg_tasks);
    	INIT_LIST_HEAD(&cset->task_iters);
    	INIT_HLIST_NODE(&cset->hlist);
    	INIT_LIST_HEAD(&cset->cgrp_links);
    	INIT_LIST_HEAD(&cset->mg_preload_node);
    	INIT_LIST_HEAD(&cset->mg_node);
    
    	/* Copy the set of subsystem state objects generated in
    	 * find_existing_css_set() */
    	memcpy(cset->subsys, template, sizeof(cset->subsys));
    
    	spin_lock_irq(&css_set_lock);
    	/* Add reference counts and links from the new css_set. */
    	list_for_each_entry(link, &old_cset->cgrp_links, cgrp_link) {
    		struct cgroup *c = link->cgrp;
    
    		if (c->root == cgrp->root)
    			c = cgrp;
    		link_css_set(&tmp_links, cset, c);
    	}
    
    	BUG_ON(!list_empty(&tmp_links));
    
    	css_set_count++;
    
    	/* Add @cset to the hash table */
    	key = css_set_hash(cset->subsys);
    	hash_add(css_set_table, &cset->hlist, key);
    
    	for_each_subsys(ss, ssid) {
    		struct cgroup_subsys_state *css = cset->subsys[ssid];
    
    		list_add_tail(&cset->e_cset_node[ssid],
    			      &css->cgroup->e_csets[ssid]);
    		css_get(css);
    	}
    
    	spin_unlock_irq(&css_set_lock);
    
    	return cset;
    }
    
    struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root)
    {
    	struct cgroup *root_cgrp = kf_root->kn->priv;
    
    	return root_cgrp->root;
    }
    
    static int cgroup_init_root_id(struct cgroup_root *root)
    {
    	int id;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, 0, 0, GFP_KERNEL);
    	if (id < 0)
    		return id;
    
    	root->hierarchy_id = id;
    	return 0;
    }
    
    static void cgroup_exit_root_id(struct cgroup_root *root)
    {
    	lockdep_assert_held(&cgroup_mutex);
    
    	idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
    }
    
    void cgroup_free_root(struct cgroup_root *root)
    {
    	if (root) {
    		idr_destroy(&root->cgroup_idr);
    		kfree(root);
    	}
    }
    
    static void cgroup_destroy_root(struct cgroup_root *root)
    {
    	struct cgroup *cgrp = &root->cgrp;
    	struct cgrp_cset_link *link, *tmp_link;
    
    	trace_cgroup_destroy_root(root);
    
    	cgroup_lock_and_drain_offline(&cgrp_dfl_root.cgrp);
    
    	BUG_ON(atomic_read(&root->nr_cgrps));
    	BUG_ON(!list_empty(&cgrp->self.children));
    
    	/* Rebind all subsystems back to the default hierarchy */
    	WARN_ON(rebind_subsystems(&cgrp_dfl_root, root->subsys_mask));
    
    	/*
    	 * Release all the links from cset_links to this hierarchy's
    	 * root cgroup
    	 */
    	spin_lock_irq(&css_set_lock);
    
    	list_for_each_entry_safe(link, tmp_link, &cgrp->cset_links, cset_link) {
    		list_del(&link->cset_link);
    		list_del(&link->cgrp_link);
    		kfree(link);
    	}
    
    	spin_unlock_irq(&css_set_lock);
    
    	if (!list_empty(&root->root_list)) {
    		list_del(&root->root_list);
    		cgroup_root_count--;
    	}
    
    	cgroup_exit_root_id(root);
    
    	mutex_unlock(&cgroup_mutex);
    
    	kernfs_destroy_root(root->kf_root);
    	cgroup_free_root(root);
    }
    
    /*
     * look up cgroup associated with current task's cgroup namespace on the
     * specified hierarchy
     */
    static struct cgroup *
    current_cgns_cgroup_from_root(struct cgroup_root *root)
    {
    	struct cgroup *res = NULL;
    	struct css_set *cset;
    
    	lockdep_assert_held(&css_set_lock);
    
    	rcu_read_lock();
    
    	cset = current->nsproxy->cgroup_ns->root_cset;
    	if (cset == &init_css_set) {
    		res = &root->cgrp;
    	} else {
    		struct cgrp_cset_link *link;
    
    		list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
    			struct cgroup *c = link->cgrp;
    
    			if (c->root == root) {
    				res = c;
    				break;
    			}
    		}
    	}
    	rcu_read_unlock();
    
    	BUG_ON(!res);
    	return res;
    }
    
    /* look up cgroup associated with given css_set on the specified hierarchy */
    static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
    					    struct cgroup_root *root)
    {
    	struct cgroup *res = NULL;
    
    	lockdep_assert_held(&cgroup_mutex);
    	lockdep_assert_held(&css_set_lock);
    
    	if (cset == &init_css_set) {
    		res = &root->cgrp;
    	} else {
    		struct cgrp_cset_link *link;
    
    		list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
    			struct cgroup *c = link->cgrp;
    
    			if (c->root == root) {
    				res = c;
    				break;
    			}
    		}
    	}
    
    	BUG_ON(!res);
    	return res;
    }
    
    /*
     * Return the cgroup for "task" from the given hierarchy. Must be
     * called with cgroup_mutex and css_set_lock held.
     */
    struct cgroup *task_cgroup_from_root(struct task_struct *task,
    				     struct cgroup_root *root)
    {
    	/*
    	 * No need to lock the task - since we hold cgroup_mutex the
    	 * task can't change groups, so the only thing that can happen
    	 * is that it exits and its css is set back to init_css_set.
    	 */
    	return cset_cgroup_from_root(task_css_set(task), root);
    }
    
    /*
     * A task must hold cgroup_mutex to modify cgroups.
     *
     * Any task can increment and decrement the count field without lock.
     * So in general, code holding cgroup_mutex can't rely on the count
     * field not changing.  However, if the count goes to zero, then only
     * cgroup_attach_task() can increment it again.  Because a count of zero
     * means that no tasks are currently attached, therefore there is no
     * way a task attached to that cgroup can fork (the other way to
     * increment the count).  So code holding cgroup_mutex can safely
     * assume that if the count is zero, it will stay zero. Similarly, if
     * a task holds cgroup_mutex on a cgroup with zero count, it
     * knows that the cgroup won't be removed, as cgroup_rmdir()
     * needs that mutex.
     *
     * A cgroup can only be deleted if both its 'count' of using tasks
     * is zero, and its list of 'children' cgroups is empty.  Since all
     * tasks in the system use _some_ cgroup, and since there is always at
     * least one task in the system (init, pid == 1), therefore, root cgroup
     * always has either children cgroups and/or using tasks.  So we don't
     * need a special hack to ensure that root cgroup cannot be deleted.
     *
     * P.S.  One more locking exception.  RCU is used to guard the
     * update of a tasks cgroup pointer by cgroup_attach_task()
     */
    
    static struct kernfs_syscall_ops cgroup_kf_syscall_ops;
    
    static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
    			      char *buf)
    {
    	struct cgroup_subsys *ss = cft->ss;
    
    	if (cft->ss && !(cft->flags & CFTYPE_NO_PREFIX) &&
    	    !(cgrp->root->flags & CGRP_ROOT_NOPREFIX))
    		snprintf(buf, CGROUP_FILE_NAME_MAX, "%s.%s",
    			 cgroup_on_dfl(cgrp) ? ss->name : ss->legacy_name,
    			 cft->name);
    	else
    		strncpy(buf, cft->name, CGROUP_FILE_NAME_MAX);
    	return buf;
    }
    
    /**
     * cgroup_file_mode - deduce file mode of a control file
     * @cft: the control file in question
     *
     * S_IRUGO for read, S_IWUSR for write.
     */
    static umode_t cgroup_file_mode(const struct cftype *cft)
    {
    	umode_t mode = 0;
    
    	if (cft->read_u64 || cft->read_s64 || cft->seq_show)
    		mode |= S_IRUGO;
    
    	if (cft->write_u64 || cft->write_s64 || cft->write) {
    		if (cft->flags & CFTYPE_WORLD_WRITABLE)
    			mode |= S_IWUGO;
    		else
    			mode |= S_IWUSR;
    	}
    
    	return mode;
    }
    
    /**
     * cgroup_calc_subtree_ss_mask - calculate subtree_ss_mask
     * @subtree_control: the new subtree_control mask to consider
     * @this_ss_mask: available subsystems
     *
     * On the default hierarchy, a subsystem may request other subsystems to be
     * enabled together through its ->depends_on mask.  In such cases, more
     * subsystems than specified in "cgroup.subtree_control" may be enabled.
     *
     * This function calculates which subsystems need to be enabled if
     * @subtree_control is to be applied while restricted to @this_ss_mask.
     */
    static u16 cgroup_calc_subtree_ss_mask(u16 subtree_control, u16 this_ss_mask)
    {
    	u16 cur_ss_mask = subtree_control;
    	struct cgroup_subsys *ss;
    	int ssid;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	cur_ss_mask |= cgrp_dfl_implicit_ss_mask;
    
    	while (true) {
    		u16 new_ss_mask = cur_ss_mask;
    
    		do_each_subsys_mask(ss, ssid, cur_ss_mask) {
    			new_ss_mask |= ss->depends_on;
    		} while_each_subsys_mask();
    
    		/*
    		 * Mask out subsystems which aren't available.  This can
    		 * happen only if some depended-upon subsystems were bound
    		 * to non-default hierarchies.
    		 */
    		new_ss_mask &= this_ss_mask;
    
    		if (new_ss_mask == cur_ss_mask)
    			break;
    		cur_ss_mask = new_ss_mask;
    	}
    
    	return cur_ss_mask;
    }
    
    /**
     * cgroup_kn_unlock - unlocking helper for cgroup kernfs methods
     * @kn: the kernfs_node being serviced
     *
     * This helper undoes cgroup_kn_lock_live() and should be invoked before
     * the method finishes if locking succeeded.  Note that once this function
     * returns the cgroup returned by cgroup_kn_lock_live() may become
     * inaccessible any time.  If the caller intends to continue to access the
     * cgroup, it should pin it before invoking this function.
     */
    void cgroup_kn_unlock(struct kernfs_node *kn)
    {
    	struct cgroup *cgrp;
    
    	if (kernfs_type(kn) == KERNFS_DIR)
    		cgrp = kn->priv;
    	else
    		cgrp = kn->parent->priv;
    
    	mutex_unlock(&cgroup_mutex);
    
    	kernfs_unbreak_active_protection(kn);
    	cgroup_put(cgrp);
    }
    
    /**
     * cgroup_kn_lock_live - locking helper for cgroup kernfs methods
     * @kn: the kernfs_node being serviced
     * @drain_offline: perform offline draining on the cgroup
     *
     * This helper is to be used by a cgroup kernfs method currently servicing
     * @kn.  It breaks the active protection, performs cgroup locking and
     * verifies that the associated cgroup is alive.  Returns the cgroup if
     * alive; otherwise, %NULL.  A successful return should be undone by a
     * matching cgroup_kn_unlock() invocation.  If @drain_offline is %true, the
     * cgroup is drained of offlining csses before return.
     *
     * Any cgroup kernfs method implementation which requires locking the
     * associated cgroup should use this helper.  It avoids nesting cgroup
     * locking under kernfs active protection and allows all kernfs operations
     * including self-removal.
     */
    struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn, bool drain_offline)
    {
    	struct cgroup *cgrp;
    
    	if (kernfs_type(kn) == KERNFS_DIR)
    		cgrp = kn->priv;
    	else
    		cgrp = kn->parent->priv;
    
    	/*
    	 * We're gonna grab cgroup_mutex which nests outside kernfs
    	 * active_ref.  cgroup liveliness check alone provides enough
    	 * protection against removal.  Ensure @cgrp stays accessible and
    	 * break the active_ref protection.
    	 */
    	if (!cgroup_tryget(cgrp))
    		return NULL;
    	kernfs_break_active_protection(kn);
    
    	if (drain_offline)
    		cgroup_lock_and_drain_offline(cgrp);
    	else
    		mutex_lock(&cgroup_mutex);
    
    	if (!cgroup_is_dead(cgrp))
    		return cgrp;
    
    	cgroup_kn_unlock(kn);
    	return NULL;
    }
    
    static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
    {
    	char name[CGROUP_FILE_NAME_MAX];
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	if (cft->file_offset) {
    		struct cgroup_subsys_state *css = cgroup_css(cgrp, cft->ss);
    		struct cgroup_file *cfile = (void *)css + cft->file_offset;
    
    		spin_lock_irq(&cgroup_file_kn_lock);
    		cfile->kn = NULL;
    		spin_unlock_irq(&cgroup_file_kn_lock);
    	}
    
    	kernfs_remove_by_name(cgrp->kn, cgroup_file_name(cgrp, cft, name));
    }
    
    /**
     * css_clear_dir - remove subsys files in a cgroup directory
     * @css: taget css
     */
    static void css_clear_dir(struct cgroup_subsys_state *css)
    {
    	struct cgroup *cgrp = css->cgroup;
    	struct cftype *cfts;
    
    	if (!(css->flags & CSS_VISIBLE))
    		return;
    
    	css->flags &= ~CSS_VISIBLE;
    
    	list_for_each_entry(cfts, &css->ss->cfts, node)
    		cgroup_addrm_files(css, cgrp, cfts, false);
    }
    
    /**
     * css_populate_dir - create subsys files in a cgroup directory
     * @css: target css
     *
     * On failure, no file is added.
     */
    static int css_populate_dir(struct cgroup_subsys_state *css)
    {
    	struct cgroup *cgrp = css->cgroup;
    	struct cftype *cfts, *failed_cfts;
    	int ret;
    
    	if ((css->flags & CSS_VISIBLE) || !cgrp->kn)
    		return 0;
    
    	if (!css->ss) {
    		if (cgroup_on_dfl(cgrp))
    			cfts = cgroup_base_files;
    		else
    			cfts = cgroup1_base_files;
    
    		return cgroup_addrm_files(&cgrp->self, cgrp, cfts, true);
    	}
    
    	list_for_each_entry(cfts, &css->ss->cfts, node) {
    		ret = cgroup_addrm_files(css, cgrp, cfts, true);
    		if (ret < 0) {
    			failed_cfts = cfts;
    			goto err;
    		}
    	}
    
    	css->flags |= CSS_VISIBLE;
    
    	return 0;
    err:
    	list_for_each_entry(cfts, &css->ss->cfts, node) {
    		if (cfts == failed_cfts)
    			break;
    		cgroup_addrm_files(css, cgrp, cfts, false);
    	}
    	return ret;
    }
    
    int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
    {
    	struct cgroup *dcgrp = &dst_root->cgrp;
    	struct cgroup_subsys *ss;
    	int ssid, i, ret;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	do_each_subsys_mask(ss, ssid, ss_mask) {
    		/*
    		 * If @ss has non-root csses attached to it, can't move.
    		 * If @ss is an implicit controller, it is exempt from this
    		 * rule and can be stolen.
    		 */
    		if (css_next_child(NULL, cgroup_css(&ss->root->cgrp, ss)) &&
    		    !ss->implicit_on_dfl)
    			return -EBUSY;
    
    		/* can't move between two non-dummy roots either */
    		if (ss->root != &cgrp_dfl_root && dst_root != &cgrp_dfl_root)
    			return -EBUSY;
    	} while_each_subsys_mask();
    
    	do_each_subsys_mask(ss, ssid, ss_mask) {
    		struct cgroup_root *src_root = ss->root;
    		struct cgroup *scgrp = &src_root->cgrp;
    		struct cgroup_subsys_state *css = cgroup_css(scgrp, ss);
    		struct css_set *cset;
    
    		WARN_ON(!css || cgroup_css(dcgrp, ss));
    
    		/* disable from the source */
    		src_root->subsys_mask &= ~(1 << ssid);
    		WARN_ON(cgroup_apply_control(scgrp));
    		cgroup_finalize_control(scgrp, 0);
    
    		/* rebind */
    		RCU_INIT_POINTER(scgrp->subsys[ssid], NULL);
    		rcu_assign_pointer(dcgrp->subsys[ssid], css);
    		ss->root = dst_root;
    		css->cgroup = dcgrp;
    
    		spin_lock_irq(&css_set_lock);
    		hash_for_each(css_set_table, i, cset, hlist)
    			list_move_tail(&cset->e_cset_node[ss->id],
    				       &dcgrp->e_csets[ss->id]);
    		spin_unlock_irq(&css_set_lock);
    
    		/* default hierarchy doesn't enable controllers by default */
    		dst_root->subsys_mask |= 1 << ssid;
    		if (dst_root == &cgrp_dfl_root) {
    			static_branch_enable(cgroup_subsys_on_dfl_key[ssid]);
    		} else {
    			dcgrp->subtree_control |= 1 << ssid;
    			static_branch_disable(cgroup_subsys_on_dfl_key[ssid]);
    		}
    
    		ret = cgroup_apply_control(dcgrp);
    		if (ret)
    			pr_warn("partial failure to rebind %s controller (err=%d)\n",
    				ss->name, ret);
    
    		if (ss->bind)
    			ss->bind(css);
    	} while_each_subsys_mask();
    
    	kernfs_activate(dcgrp->kn);
    	return 0;
    }
    
    int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
    		     struct kernfs_root *kf_root)
    {
    	int len = 0;
    	char *buf = NULL;
    	struct cgroup_root *kf_cgroot = cgroup_root_from_kf(kf_root);
    	struct cgroup *ns_cgroup;
    
    	buf = kmalloc(PATH_MAX, GFP_KERNEL);
    	if (!buf)
    		return -ENOMEM;
    
    	spin_lock_irq(&css_set_lock);
    	ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot);
    	len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX);
    	spin_unlock_irq(&css_set_lock);
    
    	if (len >= PATH_MAX)
    		len = -ERANGE;
    	else if (len > 0) {
    		seq_escape(sf, buf, " \t\n\\");
    		len = 0;
    	}
    	kfree(buf);
    	return len;
    }
    
    static int parse_cgroup_root_flags(char *data, unsigned int *root_flags)
    {
    	char *token;
    
    	*root_flags = 0;
    
    	if (!data)
    		return 0;
    
    	while ((token = strsep(&data, ",")) != NULL) {
    		if (!strcmp(token, "nsdelegate")) {
    			*root_flags |= CGRP_ROOT_NS_DELEGATE;
    			continue;
    		}
    
    		pr_err("cgroup2: unknown option \"%s\"\n", token);
    		return -EINVAL;
    	}
    
    	return 0;
    }
    
    static void apply_cgroup_root_flags(unsigned int root_flags)
    {
    	if (current->nsproxy->cgroup_ns == &init_cgroup_ns) {
    		if (root_flags & CGRP_ROOT_NS_DELEGATE)
    			cgrp_dfl_root.flags |= CGRP_ROOT_NS_DELEGATE;
    		else
    			cgrp_dfl_root.flags &= ~CGRP_ROOT_NS_DELEGATE;
    	}
    }
    
    static int cgroup_show_options(struct seq_file *seq, struct kernfs_root *kf_root)
    {
    	if (cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE)
    		seq_puts(seq, ",nsdelegate");
    	return 0;
    }
    
    static int cgroup_remount(struct kernfs_root *kf_root, int *flags, char *data)
    {
    	unsigned int root_flags;
    	int ret;
    
    	ret = parse_cgroup_root_flags(data, &root_flags);
    	if (ret)
    		return ret;
    
    	apply_cgroup_root_flags(root_flags);
    	return 0;
    }
    
    /*
     * To reduce the fork() overhead for systems that are not actually using
     * their cgroups capability, we don't maintain the lists running through
     * each css_set to its tasks until we see the list actually used - in other
     * words after the first mount.
     */
    static bool use_task_css_set_links __read_mostly;
    
    static void cgroup_enable_task_cg_lists(void)
    {
    	struct task_struct *p, *g;
    
    	spin_lock_irq(&css_set_lock);
    
    	if (use_task_css_set_links)
    		goto out_unlock;
    
    	use_task_css_set_links = true;
    
    	/*
    	 * We need tasklist_lock because RCU is not safe against
    	 * while_each_thread(). Besides, a forking task that has passed
    	 * cgroup_post_fork() without seeing use_task_css_set_links = 1
    	 * is not guaranteed to have its child immediately visible in the
    	 * tasklist if we walk through it with RCU.
    	 */
    	read_lock(&tasklist_lock);
    	do_each_thread(g, p) {
    		WARN_ON_ONCE(!list_empty(&p->cg_list) ||
    			     task_css_set(p) != &init_css_set);
    
    		/*
    		 * We should check if the process is exiting, otherwise
    		 * it will race with cgroup_exit() in that the list
    		 * entry won't be deleted though the process has exited.
    		 * Do it while holding siglock so that we don't end up
    		 * racing against cgroup_exit().
    		 *
    		 * Interrupts were already disabled while acquiring
    		 * the css_set_lock, so we do not need to disable it
    		 * again when acquiring the sighand->siglock here.
    		 */
    		spin_lock(&p->sighand->siglock);
    		if (!(p->flags & PF_EXITING)) {
    			struct css_set *cset = task_css_set(p);
    
    			if (!css_set_populated(cset))
    				css_set_update_populated(cset, true);
    			list_add_tail(&p->cg_list, &cset->tasks);
    			get_css_set(cset);
    			cset->nr_tasks++;
    		}
    		spin_unlock(&p->sighand->siglock);
    	} while_each_thread(g, p);
    	read_unlock(&tasklist_lock);
    out_unlock:
    	spin_unlock_irq(&css_set_lock);
    }
    
    static void init_cgroup_housekeeping(struct cgroup *cgrp)
    {
    	struct cgroup_subsys *ss;
    	int ssid;
    
    	INIT_LIST_HEAD(&cgrp->self.sibling);
    	INIT_LIST_HEAD(&cgrp->self.children);
    	INIT_LIST_HEAD(&cgrp->cset_links);
    	INIT_LIST_HEAD(&cgrp->pidlists);
    	mutex_init(&cgrp->pidlist_mutex);
    	cgrp->self.cgroup = cgrp;
    	cgrp->self.flags |= CSS_ONLINE;
    
    	for_each_subsys(ss, ssid)
    		INIT_LIST_HEAD(&cgrp->e_csets[ssid]);
    
    	init_waitqueue_head(&cgrp->offline_waitq);
    	INIT_WORK(&cgrp->release_agent_work, cgroup1_release_agent);
    }
    
    void init_cgroup_root(struct cgroup_root *root, struct cgroup_sb_opts *opts)
    {
    	struct cgroup *cgrp = &root->cgrp;
    
    	INIT_LIST_HEAD(&root->root_list);
    	atomic_set(&root->nr_cgrps, 1);
    	cgrp->root = root;
    	init_cgroup_housekeeping(cgrp);
    	idr_init(&root->cgroup_idr);
    
    	root->flags = opts->flags;
    	if (opts->release_agent)
    		strcpy(root->release_agent_path, opts->release_agent);
    	if (opts->name)
    		strcpy(root->name, opts->name);
    	if (opts->cpuset_clone_children)
    		set_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->cgrp.flags);
    }
    
    int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask, int ref_flags)
    {
    	LIST_HEAD(tmp_links);
    	struct cgroup *root_cgrp = &root->cgrp;
    	struct kernfs_syscall_ops *kf_sops;
    	struct css_set *cset;
    	int i, ret;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	ret = cgroup_idr_alloc(&root->cgroup_idr, root_cgrp, 1, 2, GFP_KERNEL);
    	if (ret < 0)
    		goto out;
    	root_cgrp->id = ret;
    	root_cgrp->ancestor_ids[0] = ret;
    
    	ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release,
    			      ref_flags, GFP_KERNEL);
    	if (ret)
    		goto out;
    
    	/*
    	 * We're accessing css_set_count without locking css_set_lock here,
    	 * but that's OK - it can only be increased by someone holding
    	 * cgroup_lock, and that's us.  Later rebinding may disable
    	 * controllers on the default hierarchy and thus create new csets,
    	 * which can't be more than the existing ones.  Allocate 2x.
    	 */
    	ret = allocate_cgrp_cset_links(2 * css_set_count, &tmp_links);
    	if (ret)
    		goto cancel_ref;
    
    	ret = cgroup_init_root_id(root);
    	if (ret)
    		goto cancel_ref;
    
    	kf_sops = root == &cgrp_dfl_root ?
    		&cgroup_kf_syscall_ops : &cgroup1_kf_syscall_ops;
    
    	root->kf_root = kernfs_create_root(kf_sops,
    					   KERNFS_ROOT_CREATE_DEACTIVATED,
    					   root_cgrp);
    	if (IS_ERR(root->kf_root)) {
    		ret = PTR_ERR(root->kf_root);
    		goto exit_root_id;
    	}
    	root_cgrp->kn = root->kf_root->kn;
    
    	ret = css_populate_dir(&root_cgrp->self);
    	if (ret)
    		goto destroy_root;
    
    	ret = rebind_subsystems(root, ss_mask);
    	if (ret)
    		goto destroy_root;
    
    	trace_cgroup_setup_root(root);
    
    	/*
    	 * There must be no failure case after here, since rebinding takes
    	 * care of subsystems' refcounts, which are explicitly dropped in
    	 * the failure exit path.
    	 */
    	list_add(&root->root_list, &cgroup_roots);
    	cgroup_root_count++;
    
    	/*
    	 * Link the root cgroup in this hierarchy into all the css_set
    	 * objects.
    	 */
    	spin_lock_irq(&css_set_lock);
    	hash_for_each(css_set_table, i, cset, hlist) {
    		link_css_set(&tmp_links, cset, root_cgrp);
    		if (css_set_populated(cset))
    			cgroup_update_populated(root_cgrp, true);
    	}
    	spin_unlock_irq(&css_set_lock);
    
    	BUG_ON(!list_empty(&root_cgrp->self.children));
    	BUG_ON(atomic_read(&root->nr_cgrps) != 1);
    
    	kernfs_activate(root_cgrp->kn);
    	ret = 0;
    	goto out;
    
    destroy_root:
    	kernfs_destroy_root(root->kf_root);
    	root->kf_root = NULL;
    exit_root_id:
    	cgroup_exit_root_id(root);
    cancel_ref:
    	percpu_ref_exit(&root_cgrp->self.refcnt);
    out:
    	free_cgrp_cset_links(&tmp_links);
    	return ret;
    }
    
    struct dentry *cgroup_do_mount(struct file_system_type *fs_type, int flags,
    			       struct cgroup_root *root, unsigned long magic,
    			       struct cgroup_namespace *ns)
    {
    	struct dentry *dentry;
    	bool new_sb;
    
    	dentry = kernfs_mount(fs_type, flags, root->kf_root, magic, &new_sb);
    
    	/*
    	 * In non-init cgroup namespace, instead of root cgroup's dentry,
    	 * we return the dentry corresponding to the cgroupns->root_cgrp.
    	 */
    	if (!IS_ERR(dentry) && ns != &init_cgroup_ns) {
    		struct dentry *nsdentry;
    		struct cgroup *cgrp;
    
    		mutex_lock(&cgroup_mutex);
    		spin_lock_irq(&css_set_lock);
    
    		cgrp = cset_cgroup_from_root(ns->root_cset, root);
    
    		spin_unlock_irq(&css_set_lock);
    		mutex_unlock(&cgroup_mutex);
    
    		nsdentry = kernfs_node_dentry(cgrp->kn, dentry->d_sb);
    		dput(dentry);
    		dentry = nsdentry;
    	}
    
    	if (IS_ERR(dentry) || !new_sb)
    		cgroup_put(&root->cgrp);
    
    	return dentry;
    }
    
    static struct dentry *cgroup_mount(struct file_system_type *fs_type,
    			 int flags, const char *unused_dev_name,
    			 void *data)
    {
    	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
    	struct dentry *dentry;
    	int ret;
    
    	get_cgroup_ns(ns);
    
    	/* Check if the caller has permission to mount. */
    	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) {
    		put_cgroup_ns(ns);
    		return ERR_PTR(-EPERM);
    	}
    
    	/*
    	 * The first time anyone tries to mount a cgroup, enable the list
    	 * linking each css_set to its tasks and fix up all existing tasks.
    	 */
    	if (!use_task_css_set_links)
    		cgroup_enable_task_cg_lists();
    
    	if (fs_type == &cgroup2_fs_type) {
    		unsigned int root_flags;
    
    		ret = parse_cgroup_root_flags(data, &root_flags);
    		if (ret) {
    			put_cgroup_ns(ns);
    			return ERR_PTR(ret);
    		}
    
    		cgrp_dfl_visible = true;
    		cgroup_get_live(&cgrp_dfl_root.cgrp);
    
    		dentry = cgroup_do_mount(&cgroup2_fs_type, flags, &cgrp_dfl_root,
    					 CGROUP2_SUPER_MAGIC, ns);
    		if (!IS_ERR(dentry))
    			apply_cgroup_root_flags(root_flags);
    	} else {
    		dentry = cgroup1_mount(&cgroup_fs_type, flags, data,
    				       CGROUP_SUPER_MAGIC, ns);
    	}
    
    	put_cgroup_ns(ns);
    	return dentry;
    }
    
    static void cgroup_kill_sb(struct super_block *sb)
    {
    	struct kernfs_root *kf_root = kernfs_root_from_sb(sb);
    	struct cgroup_root *root = cgroup_root_from_kf(kf_root);
    
    	/*
    	 * If @root doesn't have any mounts or children, start killing it.
    	 * This prevents new mounts by disabling percpu_ref_tryget_live().
    	 * cgroup_mount() may wait for @root's release.
    	 *
    	 * And don't kill the default root.
    	 */
    	if (!list_empty(&root->cgrp.self.children) ||
    	    root == &cgrp_dfl_root)
    		cgroup_put(&root->cgrp);
    	else
    		percpu_ref_kill(&root->cgrp.self.refcnt);
    
    	kernfs_kill_sb(sb);
    }
    
    struct file_system_type cgroup_fs_type = {
    	.name = "cgroup",
    	.mount = cgroup_mount,
    	.kill_sb = cgroup_kill_sb,
    	.fs_flags = FS_USERNS_MOUNT,
    };
    
    static struct file_system_type cgroup2_fs_type = {
    	.name = "cgroup2",
    	.mount = cgroup_mount,
    	.kill_sb = cgroup_kill_sb,
    	.fs_flags = FS_USERNS_MOUNT,
    };
    
    int cgroup_path_ns_locked(struct cgroup *cgrp, char *buf, size_t buflen,
    			  struct cgroup_namespace *ns)
    {
    	struct cgroup *root = cset_cgroup_from_root(ns->root_cset, cgrp->root);
    
    	return kernfs_path_from_node(cgrp->kn, root->kn, buf, buflen);
    }
    
    int cgroup_path_ns(struct cgroup *cgrp, char *buf, size_t buflen,
    		   struct cgroup_namespace *ns)
    {
    	int ret;
    
    	mutex_lock(&cgroup_mutex);
    	spin_lock_irq(&css_set_lock);
    
    	ret = cgroup_path_ns_locked(cgrp, buf, buflen, ns);
    
    	spin_unlock_irq(&css_set_lock);
    	mutex_unlock(&cgroup_mutex);
    
    	return ret;
    }
    EXPORT_SYMBOL_GPL(cgroup_path_ns);
    
    /**
     * task_cgroup_path - cgroup path of a task in the first cgroup hierarchy
     * @task: target task
     * @buf: the buffer to write the path into
     * @buflen: the length of the buffer
     *
     * Determine @task's cgroup on the first (the one with the lowest non-zero
     * hierarchy_id) cgroup hierarchy and copy its path into @buf.  This
     * function grabs cgroup_mutex and shouldn't be used inside locks used by
     * cgroup controller callbacks.
     *
     * Return value is the same as kernfs_path().
     */
    int task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
    {
    	struct cgroup_root *root;
    	struct cgroup *cgrp;
    	int hierarchy_id = 1;
    	int ret;
    
    	mutex_lock(&cgroup_mutex);
    	spin_lock_irq(&css_set_lock);
    
    	root = idr_get_next(&cgroup_hierarchy_idr, &hierarchy_id);
    
    	if (root) {
    		cgrp = task_cgroup_from_root(task, root);
    		ret = cgroup_path_ns_locked(cgrp, buf, buflen, &init_cgroup_ns);
    	} else {
    		/* if no hierarchy exists, everyone is in "/" */
    		ret = strlcpy(buf, "/", buflen);
    	}
    
    	spin_unlock_irq(&css_set_lock);
    	mutex_unlock(&cgroup_mutex);
    	return ret;
    }
    EXPORT_SYMBOL_GPL(task_cgroup_path);
    
    /**
     * cgroup_migrate_add_task - add a migration target task to a migration context
     * @task: target task
     * @mgctx: target migration context
     *
     * Add @task, which is a migration target, to @mgctx->tset.  This function
     * becomes noop if @task doesn't need to be migrated.  @task's css_set
     * should have been added as a migration source and @task->cg_list will be
     * moved from the css_set's tasks list to mg_tasks one.
     */
    static void cgroup_migrate_add_task(struct task_struct *task,
    				    struct cgroup_mgctx *mgctx)
    {
    	struct css_set *cset;
    
    	lockdep_assert_held(&css_set_lock);
    
    	/* @task either already exited or can't exit until the end */
    	if (task->flags & PF_EXITING)
    		return;
    
    	/* leave @task alone if post_fork() hasn't linked it yet */
    	if (list_empty(&task->cg_list))
    		return;
    
    	cset = task_css_set(task);
    	if (!cset->mg_src_cgrp)
    		return;
    
    	mgctx->tset.nr_tasks++;
    
    	list_move_tail(&task->cg_list, &cset->mg_tasks);
    	if (list_empty(&cset->mg_node))
    		list_add_tail(&cset->mg_node,
    			      &mgctx->tset.src_csets);
    	if (list_empty(&cset->mg_dst_cset->mg_node))
    		list_add_tail(&cset->mg_dst_cset->mg_node,
    			      &mgctx->tset.dst_csets);
    }
    
    /**
     * cgroup_taskset_first - reset taskset and return the first task
     * @tset: taskset of interest
     * @dst_cssp: output variable for the destination css
     *
     * @tset iteration is initialized and the first task is returned.
     */
    struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset,
    					 struct cgroup_subsys_state **dst_cssp)
    {
    	tset->cur_cset = list_first_entry(tset->csets, struct css_set, mg_node);
    	tset->cur_task = NULL;
    
    	return cgroup_taskset_next(tset, dst_cssp);
    }
    
    /**
     * cgroup_taskset_next - iterate to the next task in taskset
     * @tset: taskset of interest
     * @dst_cssp: output variable for the destination css
     *
     * Return the next task in @tset.  Iteration must have been initialized
     * with cgroup_taskset_first().
     */
    struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset,
    					struct cgroup_subsys_state **dst_cssp)
    {
    	struct css_set *cset = tset->cur_cset;
    	struct task_struct *task = tset->cur_task;
    
    	while (&cset->mg_node != tset->csets) {
    		if (!task)
    			task = list_first_entry(&cset->mg_tasks,
    						struct task_struct, cg_list);
    		else
    			task = list_next_entry(task, cg_list);
    
    		if (&task->cg_list != &cset->mg_tasks) {
    			tset->cur_cset = cset;
    			tset->cur_task = task;
    
    			/*
    			 * This function may be called both before and
    			 * after cgroup_taskset_migrate().  The two cases
    			 * can be distinguished by looking at whether @cset
    			 * has its ->mg_dst_cset set.
    			 */
    			if (cset->mg_dst_cset)
    				*dst_cssp = cset->mg_dst_cset->subsys[tset->ssid];
    			else
    				*dst_cssp = cset->subsys[tset->ssid];
    
    			return task;
    		}
    
    		cset = list_next_entry(cset, mg_node);
    		task = NULL;
    	}
    
    	return NULL;
    }
    
    /**
     * cgroup_taskset_migrate - migrate a taskset
     * @mgctx: migration context
     *
     * Migrate tasks in @mgctx as setup by migration preparation functions.
     * This function fails iff one of the ->can_attach callbacks fails and
     * guarantees that either all or none of the tasks in @mgctx are migrated.
     * @mgctx is consumed regardless of success.
     */
    static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
    {
    	struct cgroup_taskset *tset = &mgctx->tset;
    	struct cgroup_subsys *ss;
    	struct task_struct *task, *tmp_task;
    	struct css_set *cset, *tmp_cset;
    	int ssid, failed_ssid, ret;
    
    	/* check that we can legitimately attach to the cgroup */
    	if (tset->nr_tasks) {
    		do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
    			if (ss->can_attach) {
    				tset->ssid = ssid;
    				ret = ss->can_attach(tset);
    				if (ret) {
    					failed_ssid = ssid;
    					goto out_cancel_attach;
    				}
    			}
    		} while_each_subsys_mask();
    	}
    
    	/*
    	 * Now that we're guaranteed success, proceed to move all tasks to
    	 * the new cgroup.  There are no failure cases after here, so this
    	 * is the commit point.
    	 */
    	spin_lock_irq(&css_set_lock);
    	list_for_each_entry(cset, &tset->src_csets, mg_node) {
    		list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list) {
    			struct css_set *from_cset = task_css_set(task);
    			struct css_set *to_cset = cset->mg_dst_cset;
    
    			get_css_set(to_cset);
    			to_cset->nr_tasks++;
    			css_set_move_task(task, from_cset, to_cset, true);
    			put_css_set_locked(from_cset);
    			from_cset->nr_tasks--;
    		}
    	}
    	spin_unlock_irq(&css_set_lock);
    
    	/*
    	 * Migration is committed, all target tasks are now on dst_csets.
    	 * Nothing is sensitive to fork() after this point.  Notify
    	 * controllers that migration is complete.
    	 */
    	tset->csets = &tset->dst_csets;
    
    	if (tset->nr_tasks) {
    		do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
    			if (ss->attach) {
    				tset->ssid = ssid;
    				ss->attach(tset);
    			}
    		} while_each_subsys_mask();
    	}
    
    	ret = 0;
    	goto out_release_tset;
    
    out_cancel_attach:
    	if (tset->nr_tasks) {
    		do_each_subsys_mask(ss, ssid, mgctx->ss_mask) {
    			if (ssid == failed_ssid)
    				break;
    			if (ss->cancel_attach) {
    				tset->ssid = ssid;
    				ss->cancel_attach(tset);
    			}
    		} while_each_subsys_mask();
    	}
    out_release_tset:
    	spin_lock_irq(&css_set_lock);
    	list_splice_init(&tset->dst_csets, &tset->src_csets);
    	list_for_each_entry_safe(cset, tmp_cset, &tset->src_csets, mg_node) {
    		list_splice_tail_init(&cset->mg_tasks, &cset->tasks);
    		list_del_init(&cset->mg_node);
    	}
    	spin_unlock_irq(&css_set_lock);
    	return ret;
    }
    
    /**
     * cgroup_may_migrate_to - verify whether a cgroup can be migration destination
     * @dst_cgrp: destination cgroup to test
     *
     * On the default hierarchy, except for the root, subtree_control must be
     * zero for migration destination cgroups with tasks so that child cgroups
     * don't compete against tasks.
     */
    bool cgroup_may_migrate_to(struct cgroup *dst_cgrp)
    {
    	return !cgroup_on_dfl(dst_cgrp) || !cgroup_parent(dst_cgrp) ||
    		!dst_cgrp->subtree_control;
    }
    
    /**
     * cgroup_migrate_finish - cleanup after attach
     * @mgctx: migration context
     *
     * Undo cgroup_migrate_add_src() and cgroup_migrate_prepare_dst().  See
     * those functions for details.
     */
    void cgroup_migrate_finish(struct cgroup_mgctx *mgctx)
    {
    	LIST_HEAD(preloaded);
    	struct css_set *cset, *tmp_cset;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	spin_lock_irq(&css_set_lock);
    
    	list_splice_tail_init(&mgctx->preloaded_src_csets, &preloaded);
    	list_splice_tail_init(&mgctx->preloaded_dst_csets, &preloaded);
    
    	list_for_each_entry_safe(cset, tmp_cset, &preloaded, mg_preload_node) {
    		cset->mg_src_cgrp = NULL;
    		cset->mg_dst_cgrp = NULL;
    		cset->mg_dst_cset = NULL;
    		list_del_init(&cset->mg_preload_node);
    		put_css_set_locked(cset);
    	}
    
    	spin_unlock_irq(&css_set_lock);
    }
    
    /**
     * cgroup_migrate_add_src - add a migration source css_set
     * @src_cset: the source css_set to add
     * @dst_cgrp: the destination cgroup
     * @mgctx: migration context
     *
     * Tasks belonging to @src_cset are about to be migrated to @dst_cgrp.  Pin
     * @src_cset and add it to @mgctx->src_csets, which should later be cleaned
     * up by cgroup_migrate_finish().
     *
     * This function may be called without holding cgroup_threadgroup_rwsem
     * even if the target is a process.  Threads may be created and destroyed
     * but as long as cgroup_mutex is not dropped, no new css_set can be put
     * into play and the preloaded css_sets are guaranteed to cover all
     * migrations.
     */
    void cgroup_migrate_add_src(struct css_set *src_cset,
    			    struct cgroup *dst_cgrp,
    			    struct cgroup_mgctx *mgctx)
    {
    	struct cgroup *src_cgrp;
    
    	lockdep_assert_held(&cgroup_mutex);
    	lockdep_assert_held(&css_set_lock);
    
    	/*
    	 * If ->dead, @src_set is associated with one or more dead cgroups
    	 * and doesn't contain any migratable tasks.  Ignore it early so
    	 * that the rest of migration path doesn't get confused by it.
    	 */
    	if (src_cset->dead)
    		return;
    
    	src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);
    
    	if (!list_empty(&src_cset->mg_preload_node))
    		return;
    
    	WARN_ON(src_cset->mg_src_cgrp);
    	WARN_ON(src_cset->mg_dst_cgrp);
    	WARN_ON(!list_empty(&src_cset->mg_tasks));
    	WARN_ON(!list_empty(&src_cset->mg_node));
    
    	src_cset->mg_src_cgrp = src_cgrp;
    	src_cset->mg_dst_cgrp = dst_cgrp;
    	get_css_set(src_cset);
    	list_add_tail(&src_cset->mg_preload_node, &mgctx->preloaded_src_csets);
    }
    
    /**
     * cgroup_migrate_prepare_dst - prepare destination css_sets for migration
     * @mgctx: migration context
     *
     * Tasks are about to be moved and all the source css_sets have been
     * preloaded to @mgctx->preloaded_src_csets.  This function looks up and
     * pins all destination css_sets, links each to its source, and append them
     * to @mgctx->preloaded_dst_csets.
     *
     * This function must be called after cgroup_migrate_add_src() has been
     * called on each migration source css_set.  After migration is performed
     * using cgroup_migrate(), cgroup_migrate_finish() must be called on
     * @mgctx.
     */
    int cgroup_migrate_prepare_dst(struct cgroup_mgctx *mgctx)
    {
    	struct css_set *src_cset, *tmp_cset;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	/* look up the dst cset for each src cset and link it to src */
    	list_for_each_entry_safe(src_cset, tmp_cset, &mgctx->preloaded_src_csets,
    				 mg_preload_node) {
    		struct css_set *dst_cset;
    		struct cgroup_subsys *ss;
    		int ssid;
    
    		dst_cset = find_css_set(src_cset, src_cset->mg_dst_cgrp);
    		if (!dst_cset)
    			goto err;
    
    		WARN_ON_ONCE(src_cset->mg_dst_cset || dst_cset->mg_dst_cset);
    
    		/*
    		 * If src cset equals dst, it's noop.  Drop the src.
    		 * cgroup_migrate() will skip the cset too.  Note that we
    		 * can't handle src == dst as some nodes are used by both.
    		 */
    		if (src_cset == dst_cset) {
    			src_cset->mg_src_cgrp = NULL;
    			src_cset->mg_dst_cgrp = NULL;
    			list_del_init(&src_cset->mg_preload_node);
    			put_css_set(src_cset);
    			put_css_set(dst_cset);
    			continue;
    		}
    
    		src_cset->mg_dst_cset = dst_cset;
    
    		if (list_empty(&dst_cset->mg_preload_node))
    			list_add_tail(&dst_cset->mg_preload_node,
    				      &mgctx->preloaded_dst_csets);
    		else
    			put_css_set(dst_cset);
    
    		for_each_subsys(ss, ssid)
    			if (src_cset->subsys[ssid] != dst_cset->subsys[ssid])
    				mgctx->ss_mask |= 1 << ssid;
    	}
    
    	return 0;
    err:
    	cgroup_migrate_finish(mgctx);
    	return -ENOMEM;
    }
    
    /**
     * cgroup_migrate - migrate a process or task to a cgroup
     * @leader: the leader of the process or the task to migrate
     * @threadgroup: whether @leader points to the whole process or a single task
     * @mgctx: migration context
     *
     * Migrate a process or task denoted by @leader.  If migrating a process,
     * the caller must be holding cgroup_threadgroup_rwsem.  The caller is also
     * responsible for invoking cgroup_migrate_add_src() and
     * cgroup_migrate_prepare_dst() on the targets before invoking this
     * function and following up with cgroup_migrate_finish().
     *
     * As long as a controller's ->can_attach() doesn't fail, this function is
     * guaranteed to succeed.  This means that, excluding ->can_attach()
     * failure, when migrating multiple targets, the success or failure can be
     * decided for all targets by invoking group_migrate_prepare_dst() before
     * actually starting migrating.
     */
    int cgroup_migrate(struct task_struct *leader, bool threadgroup,
    		   struct cgroup_mgctx *mgctx)
    {
    	struct task_struct *task;
    
    	/*
    	 * Prevent freeing of tasks while we take a snapshot. Tasks that are
    	 * already PF_EXITING could be freed from underneath us unless we
    	 * take an rcu_read_lock.
    	 */
    	spin_lock_irq(&css_set_lock);
    	rcu_read_lock();
    	task = leader;
    	do {
    		cgroup_migrate_add_task(task, mgctx);
    		if (!threadgroup)
    			break;
    	} while_each_thread(leader, task);
    	rcu_read_unlock();
    	spin_unlock_irq(&css_set_lock);
    
    	return cgroup_migrate_execute(mgctx);
    }
    
    /**
     * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
     * @dst_cgrp: the cgroup to attach to
     * @leader: the task or the leader of the threadgroup to be attached
     * @threadgroup: attach the whole threadgroup?
     *
     * Call holding cgroup_mutex and cgroup_threadgroup_rwsem.
     */
    int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
    		       bool threadgroup)
    {
    	DEFINE_CGROUP_MGCTX(mgctx);
    	struct task_struct *task;
    	int ret;
    
    	if (!cgroup_may_migrate_to(dst_cgrp))
    		return -EBUSY;
    
    	/* look up all src csets */
    	spin_lock_irq(&css_set_lock);
    	rcu_read_lock();
    	task = leader;
    	do {
    		cgroup_migrate_add_src(task_css_set(task), dst_cgrp, &mgctx);
    		if (!threadgroup)
    			break;
    	} while_each_thread(leader, task);
    	rcu_read_unlock();
    	spin_unlock_irq(&css_set_lock);
    
    	/* prepare dst csets and commit */
    	ret = cgroup_migrate_prepare_dst(&mgctx);
    	if (!ret)
    		ret = cgroup_migrate(leader, threadgroup, &mgctx);
    
    	cgroup_migrate_finish(&mgctx);
    
    	if (!ret)
    		trace_cgroup_attach_task(dst_cgrp, leader, threadgroup);
    
    	return ret;
    }
    
    static int cgroup_procs_write_permission(struct task_struct *task,
    					 struct cgroup *dst_cgrp,
    					 struct kernfs_open_file *of)
    {
    	struct super_block *sb = of->file->f_path.dentry->d_sb;
    	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
    	struct cgroup *root_cgrp = ns->root_cset->dfl_cgrp;
    	struct cgroup *src_cgrp, *com_cgrp;
    	struct inode *inode;
    	int ret;
    
    	if (!cgroup_on_dfl(dst_cgrp)) {
    		const struct cred *cred = current_cred();
    		const struct cred *tcred = get_task_cred(task);
    
    		/*
    		 * even if we're attaching all tasks in the thread group,
    		 * we only need to check permissions on one of them.
    		 */
    		if (uid_eq(cred->euid, GLOBAL_ROOT_UID) ||
    		    uid_eq(cred->euid, tcred->uid) ||
    		    uid_eq(cred->euid, tcred->suid))
    			ret = 0;
    		else
    			ret = -EACCES;
    
    		put_cred(tcred);
    		return ret;
    	}
    
    	/* find the source cgroup */
    	spin_lock_irq(&css_set_lock);
    	src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
    	spin_unlock_irq(&css_set_lock);
    
    	/* and the common ancestor */
    	com_cgrp = src_cgrp;
    	while (!cgroup_is_descendant(dst_cgrp, com_cgrp))
    		com_cgrp = cgroup_parent(com_cgrp);
    
    	/* %current should be authorized to migrate to the common ancestor */
    	inode = kernfs_get_inode(sb, com_cgrp->procs_file.kn);
    	if (!inode)
    		return -ENOMEM;
    
    	ret = inode_permission(inode, MAY_WRITE);
    	iput(inode);
    	if (ret)
    		return ret;
    
    	/*
    	 * If namespaces are delegation boundaries, %current must be able
    	 * to see both source and destination cgroups from its namespace.
    	 */
    	if ((cgrp_dfl_root.flags & CGRP_ROOT_NS_DELEGATE) &&
    	    (!cgroup_is_descendant(src_cgrp, root_cgrp) ||
    	     !cgroup_is_descendant(dst_cgrp, root_cgrp)))
    		return -ENOENT;
    
    	return 0;
    }
    
    /*
     * Find the task_struct of the task to attach by vpid and pass it along to the
     * function to attach either it or all tasks in its threadgroup. Will lock
     * cgroup_mutex and threadgroup.
     */
    ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
    			     size_t nbytes, loff_t off, bool threadgroup)
    {
    	struct task_struct *tsk;
    	struct cgroup_subsys *ss;
    	struct cgroup *cgrp;
    	pid_t pid;
    	int ssid, ret;
    
    	if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
    		return -EINVAL;
    
    	cgrp = cgroup_kn_lock_live(of->kn, false);
    	if (!cgrp)
    		return -ENODEV;
    
    	percpu_down_write(&cgroup_threadgroup_rwsem);
    	rcu_read_lock();
    	if (pid) {
    		tsk = find_task_by_vpid(pid);
    		if (!tsk) {
    			ret = -ESRCH;
    			goto out_unlock_rcu;
    		}
    	} else {
    		tsk = current;
    	}
    
    	if (threadgroup)
    		tsk = tsk->group_leader;
    
    	/*
    	 * kthreads may acquire PF_NO_SETAFFINITY during initialization.
    	 * If userland migrates such a kthread to a non-root cgroup, it can
    	 * become trapped in a cpuset, or RT kthread may be born in a
    	 * cgroup with no rt_runtime allocated.  Just say no.
    	 */
    	if (tsk->no_cgroup_migration || (tsk->flags & PF_NO_SETAFFINITY)) {
    		ret = -EINVAL;
    		goto out_unlock_rcu;
    	}
    
    	get_task_struct(tsk);
    	rcu_read_unlock();
    
    	ret = cgroup_procs_write_permission(tsk, cgrp, of);
    	if (!ret)
    		ret = cgroup_attach_task(cgrp, tsk, threadgroup);
    
    	put_task_struct(tsk);
    	goto out_unlock_threadgroup;
    
    out_unlock_rcu:
    	rcu_read_unlock();
    out_unlock_threadgroup:
    	percpu_up_write(&cgroup_threadgroup_rwsem);
    	for_each_subsys(ss, ssid)
    		if (ss->post_attach)
    			ss->post_attach();
    	cgroup_kn_unlock(of->kn);
    	return ret ?: nbytes;
    }
    
    ssize_t cgroup_procs_write(struct kernfs_open_file *of, char *buf, size_t nbytes,
    			   loff_t off)
    {
    	return __cgroup_procs_write(of, buf, nbytes, off, true);
    }
    
    static void cgroup_print_ss_mask(struct seq_file *seq, u16 ss_mask)
    {
    	struct cgroup_subsys *ss;
    	bool printed = false;
    	int ssid;
    
    	do_each_subsys_mask(ss, ssid, ss_mask) {
    		if (printed)
    			seq_putc(seq, ' ');
    		seq_printf(seq, "%s", ss->name);
    		printed = true;
    	} while_each_subsys_mask();
    	if (printed)
    		seq_putc(seq, '\n');
    }
    
    /* show controllers which are enabled from the parent */
    static int cgroup_controllers_show(struct seq_file *seq, void *v)
    {
    	struct cgroup *cgrp = seq_css(seq)->cgroup;
    
    	cgroup_print_ss_mask(seq, cgroup_control(cgrp));
    	return 0;
    }
    
    /* show controllers which are enabled for a given cgroup's children */
    static int cgroup_subtree_control_show(struct seq_file *seq, void *v)
    {
    	struct cgroup *cgrp = seq_css(seq)->cgroup;
    
    	cgroup_print_ss_mask(seq, cgrp->subtree_control);
    	return 0;
    }
    
    /**
     * cgroup_update_dfl_csses - update css assoc of a subtree in default hierarchy
     * @cgrp: root of the subtree to update csses for
     *
     * @cgrp's control masks have changed and its subtree's css associations
     * need to be updated accordingly.  This function looks up all css_sets
     * which are attached to the subtree, creates the matching updated css_sets
     * and migrates the tasks to the new ones.
     */
    static int cgroup_update_dfl_csses(struct cgroup *cgrp)
    {
    	DEFINE_CGROUP_MGCTX(mgctx);
    	struct cgroup_subsys_state *d_css;
    	struct cgroup *dsct;
    	struct css_set *src_cset;
    	int ret;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	percpu_down_write(&cgroup_threadgroup_rwsem);
    
    	/* look up all csses currently attached to @cgrp's subtree */
    	spin_lock_irq(&css_set_lock);
    	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
    		struct cgrp_cset_link *link;
    
    		list_for_each_entry(link, &dsct->cset_links, cset_link)
    			cgroup_migrate_add_src(link->cset, dsct, &mgctx);
    	}
    	spin_unlock_irq(&css_set_lock);
    
    	/* NULL dst indicates self on default hierarchy */
    	ret = cgroup_migrate_prepare_dst(&mgctx);
    	if (ret)
    		goto out_finish;
    
    	spin_lock_irq(&css_set_lock);
    	list_for_each_entry(src_cset, &mgctx.preloaded_src_csets, mg_preload_node) {
    		struct task_struct *task, *ntask;
    
    		/* all tasks in src_csets need to be migrated */
    		list_for_each_entry_safe(task, ntask, &src_cset->tasks, cg_list)
    			cgroup_migrate_add_task(task, &mgctx);
    	}
    	spin_unlock_irq(&css_set_lock);
    
    	ret = cgroup_migrate_execute(&mgctx);
    out_finish:
    	cgroup_migrate_finish(&mgctx);
    	percpu_up_write(&cgroup_threadgroup_rwsem);
    	return ret;
    }
    
    /**
     * cgroup_lock_and_drain_offline - lock cgroup_mutex and drain offlined csses
     * @cgrp: root of the target subtree
     *
     * Because css offlining is asynchronous, userland may try to re-enable a
     * controller while the previous css is still around.  This function grabs
     * cgroup_mutex and drains the previous css instances of @cgrp's subtree.
     */
    void cgroup_lock_and_drain_offline(struct cgroup *cgrp)
    	__acquires(&cgroup_mutex)
    {
    	struct cgroup *dsct;
    	struct cgroup_subsys_state *d_css;
    	struct cgroup_subsys *ss;
    	int ssid;
    
    restart:
    	mutex_lock(&cgroup_mutex);
    
    	cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
    		for_each_subsys(ss, ssid) {
    			struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
    			DEFINE_WAIT(wait);
    
    			if (!css || !percpu_ref_is_dying(&css->refcnt))
    				continue;
    
    			cgroup_get_live(dsct);
    			prepare_to_wait(&dsct->offline_waitq, &wait,
    					TASK_UNINTERRUPTIBLE);
    
    			mutex_unlock(&cgroup_mutex);
    			schedule();
    			finish_wait(&dsct->offline_waitq, &wait);
    
    			cgroup_put(dsct);
    			goto restart;
    		}
    	}
    }
    
    /**
     * cgroup_save_control - save control masks of a subtree
     * @cgrp: root of the target subtree
     *
     * Save ->subtree_control and ->subtree_ss_mask to the respective old_
     * prefixed fields for @cgrp's subtree including @cgrp itself.
     */
    static void cgroup_save_control(struct cgroup *cgrp)
    {
    	struct cgroup *dsct;
    	struct cgroup_subsys_state *d_css;
    
    	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
    		dsct->old_subtree_control = dsct->subtree_control;
    		dsct->old_subtree_ss_mask = dsct->subtree_ss_mask;
    	}
    }
    
    /**
     * cgroup_propagate_control - refresh control masks of a subtree
     * @cgrp: root of the target subtree
     *
     * For @cgrp and its subtree, ensure ->subtree_ss_mask matches
     * ->subtree_control and propagate controller availability through the
     * subtree so that descendants don't have unavailable controllers enabled.
     */
    static void cgroup_propagate_control(struct cgroup *cgrp)
    {
    	struct cgroup *dsct;
    	struct cgroup_subsys_state *d_css;
    
    	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
    		dsct->subtree_control &= cgroup_control(dsct);
    		dsct->subtree_ss_mask =
    			cgroup_calc_subtree_ss_mask(dsct->subtree_control,
    						    cgroup_ss_mask(dsct));
    	}
    }
    
    /**
     * cgroup_restore_control - restore control masks of a subtree
     * @cgrp: root of the target subtree
     *
     * Restore ->subtree_control and ->subtree_ss_mask from the respective old_
     * prefixed fields for @cgrp's subtree including @cgrp itself.
     */
    static void cgroup_restore_control(struct cgroup *cgrp)
    {
    	struct cgroup *dsct;
    	struct cgroup_subsys_state *d_css;
    
    	cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
    		dsct->subtree_control = dsct->old_subtree_control;
    		dsct->subtree_ss_mask = dsct->old_subtree_ss_mask;
    	}
    }
    
    static bool css_visible(struct cgroup_subsys_state *css)
    {
    	struct cgroup_subsys *ss = css->ss;
    	struct cgroup *cgrp = css->cgroup;
    
    	if (cgroup_control(cgrp) & (1 << ss->id))
    		return true;
    	if (!(cgroup_ss_mask(cgrp) & (1 << ss->id)))
    		return false;
    	return cgroup_on_dfl(cgrp) && ss->implicit_on_dfl;
    }
    
    /**
     * cgroup_apply_control_enable - enable or show csses according to control
     * @cgrp: root of the target subtree
     *
     * Walk @cgrp's subtree and create new csses or make the existing ones
     * visible.  A css is created invisible if it's being implicitly enabled
     * through dependency.  An invisible css is made visible when the userland
     * explicitly enables it.
     *
     * Returns 0 on success, -errno on failure.  On failure, csses which have
     * been processed already aren't cleaned up.  The caller is responsible for
     * cleaning up with cgroup_apply_control_disable().
     */
    static int cgroup_apply_control_enable(struct cgroup *cgrp)
    {
    	struct cgroup *dsct;
    	struct cgroup_subsys_state *d_css;
    	struct cgroup_subsys *ss;
    	int ssid, ret;
    
    	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
    		for_each_subsys(ss, ssid) {
    			struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
    
    			WARN_ON_ONCE(css && percpu_ref_is_dying(&css->refcnt));
    
    			if (!(cgroup_ss_mask(dsct) & (1 << ss->id)))
    				continue;
    
    			if (!css) {
    				css = css_create(dsct, ss);
    				if (IS_ERR(css))
    					return PTR_ERR(css);
    			}
    
    			if (css_visible(css)) {
    				ret = css_populate_dir(css);
    				if (ret)
    					return ret;
    			}
    		}
    	}
    
    	return 0;
    }
    
    /**
     * cgroup_apply_control_disable - kill or hide csses according to control
     * @cgrp: root of the target subtree
     *
     * Walk @cgrp's subtree and kill and hide csses so that they match
     * cgroup_ss_mask() and cgroup_visible_mask().
     *
     * A css is hidden when the userland requests it to be disabled while other
     * subsystems are still depending on it.  The css must not actively control
     * resources and be in the vanilla state if it's made visible again later.
     * Controllers which may be depended upon should provide ->css_reset() for
     * this purpose.
     */
    static void cgroup_apply_control_disable(struct cgroup *cgrp)
    {
    	struct cgroup *dsct;
    	struct cgroup_subsys_state *d_css;
    	struct cgroup_subsys *ss;
    	int ssid;
    
    	cgroup_for_each_live_descendant_post(dsct, d_css, cgrp) {
    		for_each_subsys(ss, ssid) {
    			struct cgroup_subsys_state *css = cgroup_css(dsct, ss);
    
    			WARN_ON_ONCE(css && percpu_ref_is_dying(&css->refcnt));
    
    			if (!css)
    				continue;
    
    			if (css->parent &&
    			    !(cgroup_ss_mask(dsct) & (1 << ss->id))) {
    				kill_css(css);
    			} else if (!css_visible(css)) {
    				css_clear_dir(css);
    				if (ss->css_reset)
    					ss->css_reset(css);
    			}
    		}
    	}
    }
    
    /**
     * cgroup_apply_control - apply control mask updates to the subtree
     * @cgrp: root of the target subtree
     *
     * subsystems can be enabled and disabled in a subtree using the following
     * steps.
     *
     * 1. Call cgroup_save_control() to stash the current state.
     * 2. Update ->subtree_control masks in the subtree as desired.
     * 3. Call cgroup_apply_control() to apply the changes.
     * 4. Optionally perform other related operations.
     * 5. Call cgroup_finalize_control() to finish up.
     *
     * This function implements step 3 and propagates the mask changes
     * throughout @cgrp's subtree, updates csses accordingly and perform
     * process migrations.
     */
    static int cgroup_apply_control(struct cgroup *cgrp)
    {
    	int ret;
    
    	cgroup_propagate_control(cgrp);
    
    	ret = cgroup_apply_control_enable(cgrp);
    	if (ret)
    		return ret;
    
    	/*
    	 * At this point, cgroup_e_css() results reflect the new csses
    	 * making the following cgroup_update_dfl_csses() properly update
    	 * css associations of all tasks in the subtree.
    	 */
    	ret = cgroup_update_dfl_csses(cgrp);
    	if (ret)
    		return ret;
    
    	return 0;
    }
    
    /**
     * cgroup_finalize_control - finalize control mask update
     * @cgrp: root of the target subtree
     * @ret: the result of the update
     *
     * Finalize control mask update.  See cgroup_apply_control() for more info.
     */
    static void cgroup_finalize_control(struct cgroup *cgrp, int ret)
    {
    	if (ret) {
    		cgroup_restore_control(cgrp);
    		cgroup_propagate_control(cgrp);
    	}
    
    	cgroup_apply_control_disable(cgrp);
    }
    
    /* change the enabled child controllers for a cgroup in the default hierarchy */
    static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
    					    char *buf, size_t nbytes,
    					    loff_t off)
    {
    	u16 enable = 0, disable = 0;
    	struct cgroup *cgrp, *child;
    	struct cgroup_subsys *ss;
    	char *tok;
    	int ssid, ret;
    
    	/*
    	 * Parse input - space separated list of subsystem names prefixed
    	 * with either + or -.
    	 */
    	buf = strstrip(buf);
    	while ((tok = strsep(&buf, " "))) {
    		if (tok[0] == '\0')
    			continue;
    		do_each_subsys_mask(ss, ssid, ~cgrp_dfl_inhibit_ss_mask) {
    			if (!cgroup_ssid_enabled(ssid) ||
    			    strcmp(tok + 1, ss->name))
    				continue;
    
    			if (*tok == '+') {
    				enable |= 1 << ssid;
    				disable &= ~(1 << ssid);
    			} else if (*tok == '-') {
    				disable |= 1 << ssid;
    				enable &= ~(1 << ssid);
    			} else {
    				return -EINVAL;
    			}
    			break;
    		} while_each_subsys_mask();
    		if (ssid == CGROUP_SUBSYS_COUNT)
    			return -EINVAL;
    	}
    
    	cgrp = cgroup_kn_lock_live(of->kn, true);
    	if (!cgrp)
    		return -ENODEV;
    
    	for_each_subsys(ss, ssid) {
    		if (enable & (1 << ssid)) {
    			if (cgrp->subtree_control & (1 << ssid)) {
    				enable &= ~(1 << ssid);
    				continue;
    			}
    
    			if (!(cgroup_control(cgrp) & (1 << ssid))) {
    				ret = -ENOENT;
    				goto out_unlock;
    			}
    		} else if (disable & (1 << ssid)) {
    			if (!(cgrp->subtree_control & (1 << ssid))) {
    				disable &= ~(1 << ssid);
    				continue;
    			}
    
    			/* a child has it enabled? */
    			cgroup_for_each_live_child(child, cgrp) {
    				if (child->subtree_control & (1 << ssid)) {
    					ret = -EBUSY;
    					goto out_unlock;
    				}
    			}
    		}
    	}
    
    	if (!enable && !disable) {
    		ret = 0;
    		goto out_unlock;
    	}
    
    	/*
    	 * Except for the root, subtree_control must be zero for a cgroup
    	 * with tasks so that child cgroups don't compete against tasks.
    	 */
    	if (enable && cgroup_parent(cgrp)) {
    		struct cgrp_cset_link *link;
    
    		/*
    		 * Because namespaces pin csets too, @cgrp->cset_links
    		 * might not be empty even when @cgrp is empty.  Walk and
    		 * verify each cset.
    		 */
    		spin_lock_irq(&css_set_lock);
    
    		ret = 0;
    		list_for_each_entry(link, &cgrp->cset_links, cset_link) {
    			if (css_set_populated(link->cset)) {
    				ret = -EBUSY;
    				break;
    			}
    		}
    
    		spin_unlock_irq(&css_set_lock);
    
    		if (ret)
    			goto out_unlock;
    	}
    
    	/* save and update control masks and prepare csses */
    	cgroup_save_control(cgrp);
    
    	cgrp->subtree_control |= enable;
    	cgrp->subtree_control &= ~disable;
    
    	ret = cgroup_apply_control(cgrp);
    	cgroup_finalize_control(cgrp, ret);
    	if (ret)
    		goto out_unlock;
    
    	kernfs_activate(cgrp->kn);
    out_unlock:
    	cgroup_kn_unlock(of->kn);
    	return ret ?: nbytes;
    }
    
    static int cgroup_events_show(struct seq_file *seq, void *v)
    {
    	seq_printf(seq, "populated %d\n",
    		   cgroup_is_populated(seq_css(seq)->cgroup));
    	return 0;
    }
    
    static int cgroup_file_open(struct kernfs_open_file *of)
    {
    	struct cftype *cft = of->kn->priv;
    
    	if (cft->open)
    		return cft->open(of);
    	return 0;
    }
    
    static void cgroup_file_release(struct kernfs_open_file *of)
    {
    	struct cftype *cft = of->kn->priv;
    
    	if (cft->release)
    		cft->release(of);
    }
    
    static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
    				 size_t nbytes, loff_t off)
    {
    	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
    	struct cgroup *cgrp = of->kn->parent->priv;
    	struct cftype *cft = of->kn->priv;
    	struct cgroup_subsys_state *css;
    	int ret;
    
    	/*
    	 * If namespaces are delegation boundaries, disallow writes to
    	 * files in an non-init namespace root from inside the namespace
    	 * except for the files explicitly marked delegatable -
    	 * cgroup.procs and cgroup.subtree_control.
    	 */
    	if ((cgrp->root->flags & CGRP_ROOT_NS_DELEGATE) &&
    	    !(cft->flags & CFTYPE_NS_DELEGATABLE) &&
    	    ns != &init_cgroup_ns && ns->root_cset->dfl_cgrp == cgrp)
    		return -EPERM;
    
    	if (cft->write)
    		return cft->write(of, buf, nbytes, off);
    
    	/*
    	 * kernfs guarantees that a file isn't deleted with operations in
    	 * flight, which means that the matching css is and stays alive and
    	 * doesn't need to be pinned.  The RCU locking is not necessary
    	 * either.  It's just for the convenience of using cgroup_css().
    	 */
    	rcu_read_lock();
    	css = cgroup_css(cgrp, cft->ss);
    	rcu_read_unlock();
    
    	if (cft->write_u64) {
    		unsigned long long v;
    		ret = kstrtoull(buf, 0, &v);
    		if (!ret)
    			ret = cft->write_u64(css, cft, v);
    	} else if (cft->write_s64) {
    		long long v;
    		ret = kstrtoll(buf, 0, &v);
    		if (!ret)
    			ret = cft->write_s64(css, cft, v);
    	} else {
    		ret = -EINVAL;
    	}
    
    	return ret ?: nbytes;
    }
    
    static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
    {
    	return seq_cft(seq)->seq_start(seq, ppos);
    }
    
    static void *cgroup_seqfile_next(struct seq_file *seq, void *v, loff_t *ppos)
    {
    	return seq_cft(seq)->seq_next(seq, v, ppos);
    }
    
    static void cgroup_seqfile_stop(struct seq_file *seq, void *v)
    {
    	if (seq_cft(seq)->seq_stop)
    		seq_cft(seq)->seq_stop(seq, v);
    }
    
    static int cgroup_seqfile_show(struct seq_file *m, void *arg)
    {
    	struct cftype *cft = seq_cft(m);
    	struct cgroup_subsys_state *css = seq_css(m);
    
    	if (cft->seq_show)
    		return cft->seq_show(m, arg);
    
    	if (cft->read_u64)
    		seq_printf(m, "%llu\n", cft->read_u64(css, cft));
    	else if (cft->read_s64)
    		seq_printf(m, "%lld\n", cft->read_s64(css, cft));
    	else
    		return -EINVAL;
    	return 0;
    }
    
    static struct kernfs_ops cgroup_kf_single_ops = {
    	.atomic_write_len	= PAGE_SIZE,
    	.open			= cgroup_file_open,
    	.release		= cgroup_file_release,
    	.write			= cgroup_file_write,
    	.seq_show		= cgroup_seqfile_show,
    };
    
    static struct kernfs_ops cgroup_kf_ops = {
    	.atomic_write_len	= PAGE_SIZE,
    	.open			= cgroup_file_open,
    	.release		= cgroup_file_release,
    	.write			= cgroup_file_write,
    	.seq_start		= cgroup_seqfile_start,
    	.seq_next		= cgroup_seqfile_next,
    	.seq_stop		= cgroup_seqfile_stop,
    	.seq_show		= cgroup_seqfile_show,
    };
    
    /* set uid and gid of cgroup dirs and files to that of the creator */
    static int cgroup_kn_set_ugid(struct kernfs_node *kn)
    {
    	struct iattr iattr = { .ia_valid = ATTR_UID | ATTR_GID,
    			       .ia_uid = current_fsuid(),
    			       .ia_gid = current_fsgid(), };
    
    	if (uid_eq(iattr.ia_uid, GLOBAL_ROOT_UID) &&
    	    gid_eq(iattr.ia_gid, GLOBAL_ROOT_GID))
    		return 0;
    
    	return kernfs_setattr(kn, &iattr);
    }
    
    static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
    			   struct cftype *cft)
    {
    	char name[CGROUP_FILE_NAME_MAX];
    	struct kernfs_node *kn;
    	struct lock_class_key *key = NULL;
    	int ret;
    
    #ifdef CONFIG_DEBUG_LOCK_ALLOC
    	key = &cft->lockdep_key;
    #endif
    	kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
    				  cgroup_file_mode(cft), 0, cft->kf_ops, cft,
    				  NULL, key);
    	if (IS_ERR(kn))
    		return PTR_ERR(kn);
    
    	ret = cgroup_kn_set_ugid(kn);
    	if (ret) {
    		kernfs_remove(kn);
    		return ret;
    	}
    
    	if (cft->file_offset) {
    		struct cgroup_file *cfile = (void *)css + cft->file_offset;
    
    		spin_lock_irq(&cgroup_file_kn_lock);
    		cfile->kn = kn;
    		spin_unlock_irq(&cgroup_file_kn_lock);
    	}
    
    	return 0;
    }
    
    /**
     * cgroup_addrm_files - add or remove files to a cgroup directory
     * @css: the target css
     * @cgrp: the target cgroup (usually css->cgroup)
     * @cfts: array of cftypes to be added
     * @is_add: whether to add or remove
     *
     * Depending on @is_add, add or remove files defined by @cfts on @cgrp.
     * For removals, this function never fails.
     */
    static int cgroup_addrm_files(struct cgroup_subsys_state *css,
    			      struct cgroup *cgrp, struct cftype cfts[],
    			      bool is_add)
    {
    	struct cftype *cft, *cft_end = NULL;
    	int ret = 0;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    restart:
    	for (cft = cfts; cft != cft_end && cft->name[0] != '\0'; cft++) {
    		/* does cft->flags tell us to skip this file on @cgrp? */
    		if ((cft->flags & __CFTYPE_ONLY_ON_DFL) && !cgroup_on_dfl(cgrp))
    			continue;
    		if ((cft->flags & __CFTYPE_NOT_ON_DFL) && cgroup_on_dfl(cgrp))
    			continue;
    		if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgroup_parent(cgrp))
    			continue;
    		if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgroup_parent(cgrp))
    			continue;
    
    		if (is_add) {
    			ret = cgroup_add_file(css, cgrp, cft);
    			if (ret) {
    				pr_warn("%s: failed to add %s, err=%d\n",
    					__func__, cft->name, ret);
    				cft_end = cft;
    				is_add = false;
    				goto restart;
    			}
    		} else {
    			cgroup_rm_file(cgrp, cft);
    		}
    	}
    	return ret;
    }
    
    static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
    {
    	LIST_HEAD(pending);
    	struct cgroup_subsys *ss = cfts[0].ss;
    	struct cgroup *root = &ss->root->cgrp;
    	struct cgroup_subsys_state *css;
    	int ret = 0;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	/* add/rm files for all cgroups created before */
    	css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
    		struct cgroup *cgrp = css->cgroup;
    
    		if (!(css->flags & CSS_VISIBLE))
    			continue;
    
    		ret = cgroup_addrm_files(css, cgrp, cfts, is_add);
    		if (ret)
    			break;
    	}
    
    	if (is_add && !ret)
    		kernfs_activate(root->kn);
    	return ret;
    }
    
    static void cgroup_exit_cftypes(struct cftype *cfts)
    {
    	struct cftype *cft;
    
    	for (cft = cfts; cft->name[0] != '\0'; cft++) {
    		/* free copy for custom atomic_write_len, see init_cftypes() */
    		if (cft->max_write_len && cft->max_write_len != PAGE_SIZE)
    			kfree(cft->kf_ops);
    		cft->kf_ops = NULL;
    		cft->ss = NULL;
    
    		/* revert flags set by cgroup core while adding @cfts */
    		cft->flags &= ~(__CFTYPE_ONLY_ON_DFL | __CFTYPE_NOT_ON_DFL);
    	}
    }
    
    static int cgroup_init_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
    {
    	struct cftype *cft;
    
    	for (cft = cfts; cft->name[0] != '\0'; cft++) {
    		struct kernfs_ops *kf_ops;
    
    		WARN_ON(cft->ss || cft->kf_ops);
    
    		if (cft->seq_start)
    			kf_ops = &cgroup_kf_ops;
    		else
    			kf_ops = &cgroup_kf_single_ops;
    
    		/*
    		 * Ugh... if @cft wants a custom max_write_len, we need to
    		 * make a copy of kf_ops to set its atomic_write_len.
    		 */
    		if (cft->max_write_len && cft->max_write_len != PAGE_SIZE) {
    			kf_ops = kmemdup(kf_ops, sizeof(*kf_ops), GFP_KERNEL);
    			if (!kf_ops) {
    				cgroup_exit_cftypes(cfts);
    				return -ENOMEM;
    			}
    			kf_ops->atomic_write_len = cft->max_write_len;
    		}
    
    		cft->kf_ops = kf_ops;
    		cft->ss = ss;
    	}
    
    	return 0;
    }
    
    static int cgroup_rm_cftypes_locked(struct cftype *cfts)
    {
    	lockdep_assert_held(&cgroup_mutex);
    
    	if (!cfts || !cfts[0].ss)
    		return -ENOENT;
    
    	list_del(&cfts->node);
    	cgroup_apply_cftypes(cfts, false);
    	cgroup_exit_cftypes(cfts);
    	return 0;
    }
    
    /**
     * cgroup_rm_cftypes - remove an array of cftypes from a subsystem
     * @cfts: zero-length name terminated array of cftypes
     *
     * Unregister @cfts.  Files described by @cfts are removed from all
     * existing cgroups and all future cgroups won't have them either.  This
     * function can be called anytime whether @cfts' subsys is attached or not.
     *
     * Returns 0 on successful unregistration, -ENOENT if @cfts is not
     * registered.
     */
    int cgroup_rm_cftypes(struct cftype *cfts)
    {
    	int ret;
    
    	mutex_lock(&cgroup_mutex);
    	ret = cgroup_rm_cftypes_locked(cfts);
    	mutex_unlock(&cgroup_mutex);
    	return ret;
    }
    
    /**
     * cgroup_add_cftypes - add an array of cftypes to a subsystem
     * @ss: target cgroup subsystem
     * @cfts: zero-length name terminated array of cftypes
     *
     * Register @cfts to @ss.  Files described by @cfts are created for all
     * existing cgroups to which @ss is attached and all future cgroups will
     * have them too.  This function can be called anytime whether @ss is
     * attached or not.
     *
     * Returns 0 on successful registration, -errno on failure.  Note that this
     * function currently returns 0 as long as @cfts registration is successful
     * even if some file creation attempts on existing cgroups fail.
     */
    static int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
    {
    	int ret;
    
    	if (!cgroup_ssid_enabled(ss->id))
    		return 0;
    
    	if (!cfts || cfts[0].name[0] == '\0')
    		return 0;
    
    	ret = cgroup_init_cftypes(ss, cfts);
    	if (ret)
    		return ret;
    
    	mutex_lock(&cgroup_mutex);
    
    	list_add_tail(&cfts->node, &ss->cfts);
    	ret = cgroup_apply_cftypes(cfts, true);
    	if (ret)
    		cgroup_rm_cftypes_locked(cfts);
    
    	mutex_unlock(&cgroup_mutex);
    	return ret;
    }
    
    /**
     * cgroup_add_dfl_cftypes - add an array of cftypes for default hierarchy
     * @ss: target cgroup subsystem
     * @cfts: zero-length name terminated array of cftypes
     *
     * Similar to cgroup_add_cftypes() but the added files are only used for
     * the default hierarchy.
     */
    int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
    {
    	struct cftype *cft;
    
    	for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
    		cft->flags |= __CFTYPE_ONLY_ON_DFL;
    	return cgroup_add_cftypes(ss, cfts);
    }
    
    /**
     * cgroup_add_legacy_cftypes - add an array of cftypes for legacy hierarchies
     * @ss: target cgroup subsystem
     * @cfts: zero-length name terminated array of cftypes
     *
     * Similar to cgroup_add_cftypes() but the added files are only used for
     * the legacy hierarchies.
     */
    int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
    {
    	struct cftype *cft;
    
    	for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
    		cft->flags |= __CFTYPE_NOT_ON_DFL;
    	return cgroup_add_cftypes(ss, cfts);
    }
    
    /**
     * cgroup_file_notify - generate a file modified event for a cgroup_file
     * @cfile: target cgroup_file
     *
     * @cfile must have been obtained by setting cftype->file_offset.
     */
    void cgroup_file_notify(struct cgroup_file *cfile)
    {
    	unsigned long flags;
    
    	spin_lock_irqsave(&cgroup_file_kn_lock, flags);
    	if (cfile->kn)
    		kernfs_notify(cfile->kn);
    	spin_unlock_irqrestore(&cgroup_file_kn_lock, flags);
    }
    
    /**
     * css_next_child - find the next child of a given css
     * @pos: the current position (%NULL to initiate traversal)
     * @parent: css whose children to walk
     *
     * This function returns the next child of @parent and should be called
     * under either cgroup_mutex or RCU read lock.  The only requirement is
     * that @parent and @pos are accessible.  The next sibling is guaranteed to
     * be returned regardless of their states.
     *
     * If a subsystem synchronizes ->css_online() and the start of iteration, a
     * css which finished ->css_online() is guaranteed to be visible in the
     * future iterations and will stay visible until the last reference is put.
     * A css which hasn't finished ->css_online() or already finished
     * ->css_offline() may show up during traversal.  It's each subsystem's
     * responsibility to synchronize against on/offlining.
     */
    struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos,
    					   struct cgroup_subsys_state *parent)
    {
    	struct cgroup_subsys_state *next;
    
    	cgroup_assert_mutex_or_rcu_locked();
    
    	/*
    	 * @pos could already have been unlinked from the sibling list.
    	 * Once a cgroup is removed, its ->sibling.next is no longer
    	 * updated when its next sibling changes.  CSS_RELEASED is set when
    	 * @pos is taken off list, at which time its next pointer is valid,
    	 * and, as releases are serialized, the one pointed to by the next
    	 * pointer is guaranteed to not have started release yet.  This
    	 * implies that if we observe !CSS_RELEASED on @pos in this RCU
    	 * critical section, the one pointed to by its next pointer is
    	 * guaranteed to not have finished its RCU grace period even if we
    	 * have dropped rcu_read_lock() inbetween iterations.
    	 *
    	 * If @pos has CSS_RELEASED set, its next pointer can't be
    	 * dereferenced; however, as each css is given a monotonically
    	 * increasing unique serial number and always appended to the
    	 * sibling list, the next one can be found by walking the parent's
    	 * children until the first css with higher serial number than
    	 * @pos's.  While this path can be slower, it happens iff iteration
    	 * races against release and the race window is very small.
    	 */
    	if (!pos) {
    		next = list_entry_rcu(parent->children.next, struct cgroup_subsys_state, sibling);
    	} else if (likely(!(pos->flags & CSS_RELEASED))) {
    		next = list_entry_rcu(pos->sibling.next, struct cgroup_subsys_state, sibling);
    	} else {
    		list_for_each_entry_rcu(next, &parent->children, sibling)
    			if (next->serial_nr > pos->serial_nr)
    				break;
    	}
    
    	/*
    	 * @next, if not pointing to the head, can be dereferenced and is
    	 * the next sibling.
    	 */
    	if (&next->sibling != &parent->children)
    		return next;
    	return NULL;
    }
    
    /**
     * css_next_descendant_pre - find the next descendant for pre-order walk
     * @pos: the current position (%NULL to initiate traversal)
     * @root: css whose descendants to walk
     *
     * To be used by css_for_each_descendant_pre().  Find the next descendant
     * to visit for pre-order traversal of @root's descendants.  @root is
     * included in the iteration and the first node to be visited.
     *
     * While this function requires cgroup_mutex or RCU read locking, it
     * doesn't require the whole traversal to be contained in a single critical
     * section.  This function will return the correct next descendant as long
     * as both @pos and @root are accessible and @pos is a descendant of @root.
     *
     * If a subsystem synchronizes ->css_online() and the start of iteration, a
     * css which finished ->css_online() is guaranteed to be visible in the
     * future iterations and will stay visible until the last reference is put.
     * A css which hasn't finished ->css_online() or already finished
     * ->css_offline() may show up during traversal.  It's each subsystem's
     * responsibility to synchronize against on/offlining.
     */
    struct cgroup_subsys_state *
    css_next_descendant_pre(struct cgroup_subsys_state *pos,
    			struct cgroup_subsys_state *root)
    {
    	struct cgroup_subsys_state *next;
    
    	cgroup_assert_mutex_or_rcu_locked();
    
    	/* if first iteration, visit @root */
    	if (!pos)
    		return root;
    
    	/* visit the first child if exists */
    	next = css_next_child(NULL, pos);
    	if (next)
    		return next;
    
    	/* no child, visit my or the closest ancestor's next sibling */
    	while (pos != root) {
    		next = css_next_child(pos, pos->parent);
    		if (next)
    			return next;
    		pos = pos->parent;
    	}
    
    	return NULL;
    }
    
    /**
     * css_rightmost_descendant - return the rightmost descendant of a css
     * @pos: css of interest
     *
     * Return the rightmost descendant of @pos.  If there's no descendant, @pos
     * is returned.  This can be used during pre-order traversal to skip
     * subtree of @pos.
     *
     * While this function requires cgroup_mutex or RCU read locking, it
     * doesn't require the whole traversal to be contained in a single critical
     * section.  This function will return the correct rightmost descendant as
     * long as @pos is accessible.
     */
    struct cgroup_subsys_state *
    css_rightmost_descendant(struct cgroup_subsys_state *pos)
    {
    	struct cgroup_subsys_state *last, *tmp;
    
    	cgroup_assert_mutex_or_rcu_locked();
    
    	do {
    		last = pos;
    		/* ->prev isn't RCU safe, walk ->next till the end */
    		pos = NULL;
    		css_for_each_child(tmp, last)
    			pos = tmp;
    	} while (pos);
    
    	return last;
    }
    
    static struct cgroup_subsys_state *
    css_leftmost_descendant(struct cgroup_subsys_state *pos)
    {
    	struct cgroup_subsys_state *last;
    
    	do {
    		last = pos;
    		pos = css_next_child(NULL, pos);
    	} while (pos);
    
    	return last;
    }
    
    /**
     * css_next_descendant_post - find the next descendant for post-order walk
     * @pos: the current position (%NULL to initiate traversal)
     * @root: css whose descendants to walk
     *
     * To be used by css_for_each_descendant_post().  Find the next descendant
     * to visit for post-order traversal of @root's descendants.  @root is
     * included in the iteration and the last node to be visited.
     *
     * While this function requires cgroup_mutex or RCU read locking, it
     * doesn't require the whole traversal to be contained in a single critical
     * section.  This function will return the correct next descendant as long
     * as both @pos and @cgroup are accessible and @pos is a descendant of
     * @cgroup.
     *
     * If a subsystem synchronizes ->css_online() and the start of iteration, a
     * css which finished ->css_online() is guaranteed to be visible in the
     * future iterations and will stay visible until the last reference is put.
     * A css which hasn't finished ->css_online() or already finished
     * ->css_offline() may show up during traversal.  It's each subsystem's
     * responsibility to synchronize against on/offlining.
     */
    struct cgroup_subsys_state *
    css_next_descendant_post(struct cgroup_subsys_state *pos,
    			 struct cgroup_subsys_state *root)
    {
    	struct cgroup_subsys_state *next;
    
    	cgroup_assert_mutex_or_rcu_locked();
    
    	/* if first iteration, visit leftmost descendant which may be @root */
    	if (!pos)
    		return css_leftmost_descendant(root);
    
    	/* if we visited @root, we're done */
    	if (pos == root)
    		return NULL;
    
    	/* if there's an unvisited sibling, visit its leftmost descendant */
    	next = css_next_child(pos, pos->parent);
    	if (next)
    		return css_leftmost_descendant(next);
    
    	/* no sibling left, visit parent */
    	return pos->parent;
    }
    
    /**
     * css_has_online_children - does a css have online children
     * @css: the target css
     *
     * Returns %true if @css has any online children; otherwise, %false.  This
     * function can be called from any context but the caller is responsible
     * for synchronizing against on/offlining as necessary.
     */
    bool css_has_online_children(struct cgroup_subsys_state *css)
    {
    	struct cgroup_subsys_state *child;
    	bool ret = false;
    
    	rcu_read_lock();
    	css_for_each_child(child, css) {
    		if (child->flags & CSS_ONLINE) {
    			ret = true;
    			break;
    		}
    	}
    	rcu_read_unlock();
    	return ret;
    }
    
    /**
     * css_task_iter_advance_css_set - advance a task itererator to the next css_set
     * @it: the iterator to advance
     *
     * Advance @it to the next css_set to walk.
     */
    static void css_task_iter_advance_css_set(struct css_task_iter *it)
    {
    	struct list_head *l = it->cset_pos;
    	struct cgrp_cset_link *link;
    	struct css_set *cset;
    
    	lockdep_assert_held(&css_set_lock);
    
    	/* Advance to the next non-empty css_set */
    	do {
    		l = l->next;
    		if (l == it->cset_head) {
    			it->cset_pos = NULL;
    			it->task_pos = NULL;
    			return;
    		}
    
    		if (it->ss) {
    			cset = container_of(l, struct css_set,
    					    e_cset_node[it->ss->id]);
    		} else {
    			link = list_entry(l, struct cgrp_cset_link, cset_link);
    			cset = link->cset;
    		}
    	} while (!css_set_populated(cset));
    
    	it->cset_pos = l;
    
    	if (!list_empty(&cset->tasks))
    		it->task_pos = cset->tasks.next;
    	else
    		it->task_pos = cset->mg_tasks.next;
    
    	it->tasks_head = &cset->tasks;
    	it->mg_tasks_head = &cset->mg_tasks;
    
    	/*
    	 * We don't keep css_sets locked across iteration steps and thus
    	 * need to take steps to ensure that iteration can be resumed after
    	 * the lock is re-acquired.  Iteration is performed at two levels -
    	 * css_sets and tasks in them.
    	 *
    	 * Once created, a css_set never leaves its cgroup lists, so a
    	 * pinned css_set is guaranteed to stay put and we can resume
    	 * iteration afterwards.
    	 *
    	 * Tasks may leave @cset across iteration steps.  This is resolved
    	 * by registering each iterator with the css_set currently being
    	 * walked and making css_set_move_task() advance iterators whose
    	 * next task is leaving.
    	 */
    	if (it->cur_cset) {
    		list_del(&it->iters_node);
    		put_css_set_locked(it->cur_cset);
    	}
    	get_css_set(cset);
    	it->cur_cset = cset;
    	list_add(&it->iters_node, &cset->task_iters);
    }
    
    static void css_task_iter_advance(struct css_task_iter *it)
    {
    	struct list_head *l = it->task_pos;
    
    	lockdep_assert_held(&css_set_lock);
    	WARN_ON_ONCE(!l);
    
    	/*
    	 * Advance iterator to find next entry.  cset->tasks is consumed
    	 * first and then ->mg_tasks.  After ->mg_tasks, we move onto the
    	 * next cset.
    	 */
    	l = l->next;
    
    	if (l == it->tasks_head)
    		l = it->mg_tasks_head->next;
    
    	if (l == it->mg_tasks_head)
    		css_task_iter_advance_css_set(it);
    	else
    		it->task_pos = l;
    }
    
    /**
     * css_task_iter_start - initiate task iteration
     * @css: the css to walk tasks of
     * @it: the task iterator to use
     *
     * Initiate iteration through the tasks of @css.  The caller can call
     * css_task_iter_next() to walk through the tasks until the function
     * returns NULL.  On completion of iteration, css_task_iter_end() must be
     * called.
     */
    void css_task_iter_start(struct cgroup_subsys_state *css,
    			 struct css_task_iter *it)
    {
    	/* no one should try to iterate before mounting cgroups */
    	WARN_ON_ONCE(!use_task_css_set_links);
    
    	memset(it, 0, sizeof(*it));
    
    	spin_lock_irq(&css_set_lock);
    
    	it->ss = css->ss;
    
    	if (it->ss)
    		it->cset_pos = &css->cgroup->e_csets[css->ss->id];
    	else
    		it->cset_pos = &css->cgroup->cset_links;
    
    	it->cset_head = it->cset_pos;
    
    	css_task_iter_advance_css_set(it);
    
    	spin_unlock_irq(&css_set_lock);
    }
    
    /**
     * css_task_iter_next - return the next task for the iterator
     * @it: the task iterator being iterated
     *
     * The "next" function for task iteration.  @it should have been
     * initialized via css_task_iter_start().  Returns NULL when the iteration
     * reaches the end.
     */
    struct task_struct *css_task_iter_next(struct css_task_iter *it)
    {
    	if (it->cur_task) {
    		put_task_struct(it->cur_task);
    		it->cur_task = NULL;
    	}
    
    	spin_lock_irq(&css_set_lock);
    
    	if (it->task_pos) {
    		it->cur_task = list_entry(it->task_pos, struct task_struct,
    					  cg_list);
    		get_task_struct(it->cur_task);
    		css_task_iter_advance(it);
    	}
    
    	spin_unlock_irq(&css_set_lock);
    
    	return it->cur_task;
    }
    
    /**
     * css_task_iter_end - finish task iteration
     * @it: the task iterator to finish
     *
     * Finish task iteration started by css_task_iter_start().
     */
    void css_task_iter_end(struct css_task_iter *it)
    {
    	if (it->cur_cset) {
    		spin_lock_irq(&css_set_lock);
    		list_del(&it->iters_node);
    		put_css_set_locked(it->cur_cset);
    		spin_unlock_irq(&css_set_lock);
    	}
    
    	if (it->cur_task)
    		put_task_struct(it->cur_task);
    }
    
    static void cgroup_procs_release(struct kernfs_open_file *of)
    {
    	if (of->priv) {
    		css_task_iter_end(of->priv);
    		kfree(of->priv);
    	}
    }
    
    static void *cgroup_procs_next(struct seq_file *s, void *v, loff_t *pos)
    {
    	struct kernfs_open_file *of = s->private;
    	struct css_task_iter *it = of->priv;
    	struct task_struct *task;
    
    	do {
    		task = css_task_iter_next(it);
    	} while (task && !thread_group_leader(task));
    
    	return task;
    }
    
    static void *cgroup_procs_start(struct seq_file *s, loff_t *pos)
    {
    	struct kernfs_open_file *of = s->private;
    	struct cgroup *cgrp = seq_css(s)->cgroup;
    	struct css_task_iter *it = of->priv;
    
    	/*
    	 * When a seq_file is seeked, it's always traversed sequentially
    	 * from position 0, so we can simply keep iterating on !0 *pos.
    	 */
    	if (!it) {
    		if (WARN_ON_ONCE((*pos)++))
    			return ERR_PTR(-EINVAL);
    
    		it = kzalloc(sizeof(*it), GFP_KERNEL);
    		if (!it)
    			return ERR_PTR(-ENOMEM);
    		of->priv = it;
    		css_task_iter_start(&cgrp->self, it);
    	} else if (!(*pos)++) {
    		css_task_iter_end(it);
    		css_task_iter_start(&cgrp->self, it);
    	}
    
    	return cgroup_procs_next(s, NULL, NULL);
    }
    
    static int cgroup_procs_show(struct seq_file *s, void *v)
    {
    	seq_printf(s, "%d\n", task_tgid_vnr(v));
    	return 0;
    }
    
    /* cgroup core interface files for the default hierarchy */
    static struct cftype cgroup_base_files[] = {
    	{
    		.name = "cgroup.procs",
    		.flags = CFTYPE_NS_DELEGATABLE,
    		.file_offset = offsetof(struct cgroup, procs_file),
    		.release = cgroup_procs_release,
    		.seq_start = cgroup_procs_start,
    		.seq_next = cgroup_procs_next,
    		.seq_show = cgroup_procs_show,
    		.write = cgroup_procs_write,
    	},
    	{
    		.name = "cgroup.controllers",
    		.seq_show = cgroup_controllers_show,
    	},
    	{
    		.name = "cgroup.subtree_control",
    		.flags = CFTYPE_NS_DELEGATABLE,
    		.seq_show = cgroup_subtree_control_show,
    		.write = cgroup_subtree_control_write,
    	},
    	{
    		.name = "cgroup.events",
    		.flags = CFTYPE_NOT_ON_ROOT,
    		.file_offset = offsetof(struct cgroup, events_file),
    		.seq_show = cgroup_events_show,
    	},
    	{ }	/* terminate */
    };
    
    /*
     * css destruction is four-stage process.
     *
     * 1. Destruction starts.  Killing of the percpu_ref is initiated.
     *    Implemented in kill_css().
     *
     * 2. When the percpu_ref is confirmed to be visible as killed on all CPUs
     *    and thus css_tryget_online() is guaranteed to fail, the css can be
     *    offlined by invoking offline_css().  After offlining, the base ref is
     *    put.  Implemented in css_killed_work_fn().
     *
     * 3. When the percpu_ref reaches zero, the only possible remaining
     *    accessors are inside RCU read sections.  css_release() schedules the
     *    RCU callback.
     *
     * 4. After the grace period, the css can be freed.  Implemented in
     *    css_free_work_fn().
     *
     * It is actually hairier because both step 2 and 4 require process context
     * and thus involve punting to css->destroy_work adding two additional
     * steps to the already complex sequence.
     */
    static void css_free_work_fn(struct work_struct *work)
    {
    	struct cgroup_subsys_state *css =
    		container_of(work, struct cgroup_subsys_state, destroy_work);
    	struct cgroup_subsys *ss = css->ss;
    	struct cgroup *cgrp = css->cgroup;
    
    	percpu_ref_exit(&css->refcnt);
    
    	if (ss) {
    		/* css free path */
    		struct cgroup_subsys_state *parent = css->parent;
    		int id = css->id;
    
    		ss->css_free(css);
    		cgroup_idr_remove(&ss->css_idr, id);
    		cgroup_put(cgrp);
    
    		if (parent)
    			css_put(parent);
    	} else {
    		/* cgroup free path */
    		atomic_dec(&cgrp->root->nr_cgrps);
    		cgroup1_pidlist_destroy_all(cgrp);
    		cancel_work_sync(&cgrp->release_agent_work);
    
    		if (cgroup_parent(cgrp)) {
    			/*
    			 * We get a ref to the parent, and put the ref when
    			 * this cgroup is being freed, so it's guaranteed
    			 * that the parent won't be destroyed before its
    			 * children.
    			 */
    			cgroup_put(cgroup_parent(cgrp));
    			kernfs_put(cgrp->kn);
    			kfree(cgrp);
    		} else {
    			/*
    			 * This is root cgroup's refcnt reaching zero,
    			 * which indicates that the root should be
    			 * released.
    			 */
    			cgroup_destroy_root(cgrp->root);
    		}
    	}
    }
    
    static void css_free_rcu_fn(struct rcu_head *rcu_head)
    {
    	struct cgroup_subsys_state *css =
    		container_of(rcu_head, struct cgroup_subsys_state, rcu_head);
    
    	INIT_WORK(&css->destroy_work, css_free_work_fn);
    	queue_work(cgroup_destroy_wq, &css->destroy_work);
    }
    
    static void css_release_work_fn(struct work_struct *work)
    {
    	struct cgroup_subsys_state *css =
    		container_of(work, struct cgroup_subsys_state, destroy_work);
    	struct cgroup_subsys *ss = css->ss;
    	struct cgroup *cgrp = css->cgroup;
    
    	mutex_lock(&cgroup_mutex);
    
    	css->flags |= CSS_RELEASED;
    	list_del_rcu(&css->sibling);
    
    	if (ss) {
    		/* css release path */
    		cgroup_idr_replace(&ss->css_idr, NULL, css->id);
    		if (ss->css_released)
    			ss->css_released(css);
    	} else {
    		/* cgroup release path */
    		trace_cgroup_release(cgrp);
    
    		cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
    		cgrp->id = -1;
    
    		/*
    		 * There are two control paths which try to determine
    		 * cgroup from dentry without going through kernfs -
    		 * cgroupstats_build() and css_tryget_online_from_dir().
    		 * Those are supported by RCU protecting clearing of
    		 * cgrp->kn->priv backpointer.
    		 */
    		if (cgrp->kn)
    			RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv,
    					 NULL);
    
    		cgroup_bpf_put(cgrp);
    	}
    
    	mutex_unlock(&cgroup_mutex);
    
    	call_rcu(&css->rcu_head, css_free_rcu_fn);
    }
    
    static void css_release(struct percpu_ref *ref)
    {
    	struct cgroup_subsys_state *css =
    		container_of(ref, struct cgroup_subsys_state, refcnt);
    
    	INIT_WORK(&css->destroy_work, css_release_work_fn);
    	queue_work(cgroup_destroy_wq, &css->destroy_work);
    }
    
    static void init_and_link_css(struct cgroup_subsys_state *css,
    			      struct cgroup_subsys *ss, struct cgroup *cgrp)
    {
    	lockdep_assert_held(&cgroup_mutex);
    
    	cgroup_get_live(cgrp);
    
    	memset(css, 0, sizeof(*css));
    	css->cgroup = cgrp;
    	css->ss = ss;
    	css->id = -1;
    	INIT_LIST_HEAD(&css->sibling);
    	INIT_LIST_HEAD(&css->children);
    	css->serial_nr = css_serial_nr_next++;
    	atomic_set(&css->online_cnt, 0);
    
    	if (cgroup_parent(cgrp)) {
    		css->parent = cgroup_css(cgroup_parent(cgrp), ss);
    		css_get(css->parent);
    	}
    
    	BUG_ON(cgroup_css(cgrp, ss));
    }
    
    /* invoke ->css_online() on a new CSS and mark it online if successful */
    static int online_css(struct cgroup_subsys_state *css)
    {
    	struct cgroup_subsys *ss = css->ss;
    	int ret = 0;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	if (ss->css_online)
    		ret = ss->css_online(css);
    	if (!ret) {
    		css->flags |= CSS_ONLINE;
    		rcu_assign_pointer(css->cgroup->subsys[ss->id], css);
    
    		atomic_inc(&css->online_cnt);
    		if (css->parent)
    			atomic_inc(&css->parent->online_cnt);
    	}
    	return ret;
    }
    
    /* if the CSS is online, invoke ->css_offline() on it and mark it offline */
    static void offline_css(struct cgroup_subsys_state *css)
    {
    	struct cgroup_subsys *ss = css->ss;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	if (!(css->flags & CSS_ONLINE))
    		return;
    
    	if (ss->css_offline)
    		ss->css_offline(css);
    
    	css->flags &= ~CSS_ONLINE;
    	RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);
    
    	wake_up_all(&css->cgroup->offline_waitq);
    }
    
    /**
     * css_create - create a cgroup_subsys_state
     * @cgrp: the cgroup new css will be associated with
     * @ss: the subsys of new css
     *
     * Create a new css associated with @cgrp - @ss pair.  On success, the new
     * css is online and installed in @cgrp.  This function doesn't create the
     * interface files.  Returns 0 on success, -errno on failure.
     */
    static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
    					      struct cgroup_subsys *ss)
    {
    	struct cgroup *parent = cgroup_parent(cgrp);
    	struct cgroup_subsys_state *parent_css = cgroup_css(parent, ss);
    	struct cgroup_subsys_state *css;
    	int err;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	css = ss->css_alloc(parent_css);
    	if (!css)
    		css = ERR_PTR(-ENOMEM);
    	if (IS_ERR(css))
    		return css;
    
    	init_and_link_css(css, ss, cgrp);
    
    	err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
    	if (err)
    		goto err_free_css;
    
    	err = cgroup_idr_alloc(&ss->css_idr, NULL, 2, 0, GFP_KERNEL);
    	if (err < 0)
    		goto err_free_css;
    	css->id = err;
    
    	/* @css is ready to be brought online now, make it visible */
    	list_add_tail_rcu(&css->sibling, &parent_css->children);
    	cgroup_idr_replace(&ss->css_idr, css, css->id);
    
    	err = online_css(css);
    	if (err)
    		goto err_list_del;
    
    	if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
    	    cgroup_parent(parent)) {
    		pr_warn("%s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
    			current->comm, current->pid, ss->name);
    		if (!strcmp(ss->name, "memory"))
    			pr_warn("\"memory\" requires setting use_hierarchy to 1 on the root\n");
    		ss->warned_broken_hierarchy = true;
    	}
    
    	return css;
    
    err_list_del:
    	list_del_rcu(&css->sibling);
    err_free_css:
    	call_rcu(&css->rcu_head, css_free_rcu_fn);
    	return ERR_PTR(err);
    }
    
    /*
     * The returned cgroup is fully initialized including its control mask, but
     * it isn't associated with its kernfs_node and doesn't have the control
     * mask applied.
     */
    static struct cgroup *cgroup_create(struct cgroup *parent)
    {
    	struct cgroup_root *root = parent->root;
    	struct cgroup *cgrp, *tcgrp;
    	int level = parent->level + 1;
    	int ret;
    
    	/* allocate the cgroup and its ID, 0 is reserved for the root */
    	cgrp = kzalloc(sizeof(*cgrp) +
    		       sizeof(cgrp->ancestor_ids[0]) * (level + 1), GFP_KERNEL);
    	if (!cgrp)
    		return ERR_PTR(-ENOMEM);
    
    	ret = percpu_ref_init(&cgrp->self.refcnt, css_release, 0, GFP_KERNEL);
    	if (ret)
    		goto out_free_cgrp;
    
    	/*
    	 * Temporarily set the pointer to NULL, so idr_find() won't return
    	 * a half-baked cgroup.
    	 */
    	cgrp->id = cgroup_idr_alloc(&root->cgroup_idr, NULL, 2, 0, GFP_KERNEL);
    	if (cgrp->id < 0) {
    		ret = -ENOMEM;
    		goto out_cancel_ref;
    	}
    
    	init_cgroup_housekeeping(cgrp);
    
    	cgrp->self.parent = &parent->self;
    	cgrp->root = root;
    	cgrp->level = level;
    
    	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp))
    		cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;
    
    	if (notify_on_release(parent))
    		set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
    
    	if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &parent->flags))
    		set_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);
    
    	cgrp->self.serial_nr = css_serial_nr_next++;
    
    	/* allocation complete, commit to creation */
    	list_add_tail_rcu(&cgrp->self.sibling, &cgroup_parent(cgrp)->self.children);
    	atomic_inc(&root->nr_cgrps);
    	cgroup_get_live(parent);
    
    	/*
    	 * @cgrp is now fully operational.  If something fails after this
    	 * point, it'll be released via the normal destruction path.
    	 */
    	cgroup_idr_replace(&root->cgroup_idr, cgrp, cgrp->id);
    
    	/*
    	 * On the default hierarchy, a child doesn't automatically inherit
    	 * subtree_control from the parent.  Each is configured manually.
    	 */
    	if (!cgroup_on_dfl(cgrp))
    		cgrp->subtree_control = cgroup_control(cgrp);
    
    	if (parent)
    		cgroup_bpf_inherit(cgrp, parent);
    
    	cgroup_propagate_control(cgrp);
    
    	return cgrp;
    
    out_cancel_ref:
    	percpu_ref_exit(&cgrp->self.refcnt);
    out_free_cgrp:
    	kfree(cgrp);
    	return ERR_PTR(ret);
    }
    
    int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
    {
    	struct cgroup *parent, *cgrp;
    	struct kernfs_node *kn;
    	int ret;
    
    	/* do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable */
    	if (strchr(name, '\n'))
    		return -EINVAL;
    
    	parent = cgroup_kn_lock_live(parent_kn, false);
    	if (!parent)
    		return -ENODEV;
    
    	cgrp = cgroup_create(parent);
    	if (IS_ERR(cgrp)) {
    		ret = PTR_ERR(cgrp);
    		goto out_unlock;
    	}
    
    	/* create the directory */
    	kn = kernfs_create_dir(parent->kn, name, mode, cgrp);
    	if (IS_ERR(kn)) {
    		ret = PTR_ERR(kn);
    		goto out_destroy;
    	}
    	cgrp->kn = kn;
    
    	/*
    	 * This extra ref will be put in cgroup_free_fn() and guarantees
    	 * that @cgrp->kn is always accessible.
    	 */
    	kernfs_get(kn);
    
    	ret = cgroup_kn_set_ugid(kn);
    	if (ret)
    		goto out_destroy;
    
    	ret = css_populate_dir(&cgrp->self);
    	if (ret)
    		goto out_destroy;
    
    	ret = cgroup_apply_control_enable(cgrp);
    	if (ret)
    		goto out_destroy;
    
    	trace_cgroup_mkdir(cgrp);
    
    	/* let's create and online css's */
    	kernfs_activate(kn);
    
    	ret = 0;
    	goto out_unlock;
    
    out_destroy:
    	cgroup_destroy_locked(cgrp);
    out_unlock:
    	cgroup_kn_unlock(parent_kn);
    	return ret;
    }
    
    /*
     * This is called when the refcnt of a css is confirmed to be killed.
     * css_tryget_online() is now guaranteed to fail.  Tell the subsystem to
     * initate destruction and put the css ref from kill_css().
     */
    static void css_killed_work_fn(struct work_struct *work)
    {
    	struct cgroup_subsys_state *css =
    		container_of(work, struct cgroup_subsys_state, destroy_work);
    
    	mutex_lock(&cgroup_mutex);
    
    	do {
    		offline_css(css);
    		css_put(css);
    		/* @css can't go away while we're holding cgroup_mutex */
    		css = css->parent;
    	} while (css && atomic_dec_and_test(&css->online_cnt));
    
    	mutex_unlock(&cgroup_mutex);
    }
    
    /* css kill confirmation processing requires process context, bounce */
    static void css_killed_ref_fn(struct percpu_ref *ref)
    {
    	struct cgroup_subsys_state *css =
    		container_of(ref, struct cgroup_subsys_state, refcnt);
    
    	if (atomic_dec_and_test(&css->online_cnt)) {
    		INIT_WORK(&css->destroy_work, css_killed_work_fn);
    		queue_work(cgroup_destroy_wq, &css->destroy_work);
    	}
    }
    
    /**
     * kill_css - destroy a css
     * @css: css to destroy
     *
     * This function initiates destruction of @css by removing cgroup interface
     * files and putting its base reference.  ->css_offline() will be invoked
     * asynchronously once css_tryget_online() is guaranteed to fail and when
     * the reference count reaches zero, @css will be released.
     */
    static void kill_css(struct cgroup_subsys_state *css)
    {
    	lockdep_assert_held(&cgroup_mutex);
    
    	if (css->flags & CSS_DYING)
    		return;
    
    	css->flags |= CSS_DYING;
    
    	/*
    	 * This must happen before css is disassociated with its cgroup.
    	 * See seq_css() for details.
    	 */
    	css_clear_dir(css);
    
    	/*
    	 * Killing would put the base ref, but we need to keep it alive
    	 * until after ->css_offline().
    	 */
    	css_get(css);
    
    	/*
    	 * cgroup core guarantees that, by the time ->css_offline() is
    	 * invoked, no new css reference will be given out via
    	 * css_tryget_online().  We can't simply call percpu_ref_kill() and
    	 * proceed to offlining css's because percpu_ref_kill() doesn't
    	 * guarantee that the ref is seen as killed on all CPUs on return.
    	 *
    	 * Use percpu_ref_kill_and_confirm() to get notifications as each
    	 * css is confirmed to be seen as killed on all CPUs.
    	 */
    	percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
    }
    
    /**
     * cgroup_destroy_locked - the first stage of cgroup destruction
     * @cgrp: cgroup to be destroyed
     *
     * css's make use of percpu refcnts whose killing latency shouldn't be
     * exposed to userland and are RCU protected.  Also, cgroup core needs to
     * guarantee that css_tryget_online() won't succeed by the time
     * ->css_offline() is invoked.  To satisfy all the requirements,
     * destruction is implemented in the following two steps.
     *
     * s1. Verify @cgrp can be destroyed and mark it dying.  Remove all
     *     userland visible parts and start killing the percpu refcnts of
     *     css's.  Set up so that the next stage will be kicked off once all
     *     the percpu refcnts are confirmed to be killed.
     *
     * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the
     *     rest of destruction.  Once all cgroup references are gone, the
     *     cgroup is RCU-freed.
     *
     * This function implements s1.  After this step, @cgrp is gone as far as
     * the userland is concerned and a new cgroup with the same name may be
     * created.  As cgroup doesn't care about the names internally, this
     * doesn't cause any problem.
     */
    static int cgroup_destroy_locked(struct cgroup *cgrp)
    	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
    {
    	struct cgroup_subsys_state *css;
    	struct cgrp_cset_link *link;
    	int ssid;
    
    	lockdep_assert_held(&cgroup_mutex);
    
    	/*
    	 * Only migration can raise populated from zero and we're already
    	 * holding cgroup_mutex.
    	 */
    	if (cgroup_is_populated(cgrp))
    		return -EBUSY;
    
    	/*
    	 * Make sure there's no live children.  We can't test emptiness of
    	 * ->self.children as dead children linger on it while being
    	 * drained; otherwise, "rmdir parent/child parent" may fail.
    	 */
    	if (css_has_online_children(&cgrp->self))
    		return -EBUSY;
    
    	/*
    	 * Mark @cgrp and the associated csets dead.  The former prevents
    	 * further task migration and child creation by disabling
    	 * cgroup_lock_live_group().  The latter makes the csets ignored by
    	 * the migration path.
    	 */
    	cgrp->self.flags &= ~CSS_ONLINE;
    
    	spin_lock_irq(&css_set_lock);
    	list_for_each_entry(link, &cgrp->cset_links, cset_link)
    		link->cset->dead = true;
    	spin_unlock_irq(&css_set_lock);
    
    	/* initiate massacre of all css's */
    	for_each_css(css, ssid, cgrp)
    		kill_css(css);
    
    	/*
    	 * Remove @cgrp directory along with the base files.  @cgrp has an
    	 * extra ref on its kn.
    	 */
    	kernfs_remove(cgrp->kn);
    
    	cgroup1_check_for_release(cgroup_parent(cgrp));
    
    	/* put the base reference */
    	percpu_ref_kill(&cgrp->self.refcnt);
    
    	return 0;
    };
    
    int cgroup_rmdir(struct kernfs_node *kn)
    {
    	struct cgroup *cgrp;
    	int ret = 0;
    
    	cgrp = cgroup_kn_lock_live(kn, false);
    	if (!cgrp)
    		return 0;
    
    	ret = cgroup_destroy_locked(cgrp);
    
    	if (!ret)
    		trace_cgroup_rmdir(cgrp);
    
    	cgroup_kn_unlock(kn);
    	return ret;
    }
    
    static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
    	.show_options		= cgroup_show_options,
    	.remount_fs		= cgroup_remount,
    	.mkdir			= cgroup_mkdir,
    	.rmdir			= cgroup_rmdir,
    	.show_path		= cgroup_show_path,
    };
    
    static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
    {
    	struct cgroup_subsys_state *css;
    
    	pr_debug("Initializing cgroup subsys %s\n", ss->name);
    
    	mutex_lock(&cgroup_mutex);
    
    	idr_init(&ss->css_idr);
    	INIT_LIST_HEAD(&ss->cfts);
    
    	/* Create the root cgroup state for this subsystem */
    	ss->root = &cgrp_dfl_root;
    	css = ss->css_alloc(cgroup_css(&cgrp_dfl_root.cgrp, ss));
    	/* We don't handle early failures gracefully */
    	BUG_ON(IS_ERR(css));
    	init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);
    
    	/*
    	 * Root csses are never destroyed and we can't initialize
    	 * percpu_ref during early init.  Disable refcnting.
    	 */
    	css->flags |= CSS_NO_REF;
    
    	if (early) {
    		/* allocation can't be done safely during early init */
    		css->id = 1;
    	} else {
    		css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2, GFP_KERNEL);
    		BUG_ON(css->id < 0);
    	}
    
    	/* Update the init_css_set to contain a subsys
    	 * pointer to this state - since the subsystem is
    	 * newly registered, all tasks and hence the
    	 * init_css_set is in the subsystem's root cgroup. */
    	init_css_set.subsys[ss->id] = css;
    
    	have_fork_callback |= (bool)ss->fork << ss->id;
    	have_exit_callback |= (bool)ss->exit << ss->id;
    	have_free_callback |= (bool)ss->free << ss->id;
    	have_canfork_callback |= (bool)ss->can_fork << ss->id;
    
    	/* At system boot, before all subsystems have been
    	 * registered, no tasks have been forked, so we don't
    	 * need to invoke fork callbacks here. */
    	BUG_ON(!list_empty(&init_task.tasks));
    
    	BUG_ON(online_css(css));
    
    	mutex_unlock(&cgroup_mutex);
    }
    
    /**
     * cgroup_init_early - cgroup initialization at system boot
     *
     * Initialize cgroups at system boot, and initialize any
     * subsystems that request early init.
     */
    int __init cgroup_init_early(void)
    {
    	static struct cgroup_sb_opts __initdata opts;
    	struct cgroup_subsys *ss;
    	int i;
    
    	init_cgroup_root(&cgrp_dfl_root, &opts);
    	cgrp_dfl_root.cgrp.self.flags |= CSS_NO_REF;
    
    	RCU_INIT_POINTER(init_task.cgroups, &init_css_set);
    
    	for_each_subsys(ss, i) {
    		WARN(!ss->css_alloc || !ss->css_free || ss->name || ss->id,
    		     "invalid cgroup_subsys %d:%s css_alloc=%p css_free=%p id:name=%d:%s\n",
    		     i, cgroup_subsys_name[i], ss->css_alloc, ss->css_free,
    		     ss->id, ss->name);
    		WARN(strlen(cgroup_subsys_name[i]) > MAX_CGROUP_TYPE_NAMELEN,
    		     "cgroup_subsys_name %s too long\n", cgroup_subsys_name[i]);
    
    		ss->id = i;
    		ss->name = cgroup_subsys_name[i];
    		if (!ss->legacy_name)
    			ss->legacy_name = cgroup_subsys_name[i];
    
    		if (ss->early_init)
    			cgroup_init_subsys(ss, true);
    	}
    	return 0;
    }
    
    static u16 cgroup_disable_mask __initdata;
    
    /**
     * cgroup_init - cgroup initialization
     *
     * Register cgroup filesystem and /proc file, and initialize
     * any subsystems that didn't request early init.
     */
    int __init cgroup_init(void)
    {
    	struct cgroup_subsys *ss;
    	int ssid;
    
    	BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16);
    	BUG_ON(percpu_init_rwsem(&cgroup_threadgroup_rwsem));
    	BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));
    	BUG_ON(cgroup_init_cftypes(NULL, cgroup1_base_files));
    
    	/*
    	 * The latency of the synchronize_sched() is too high for cgroups,
    	 * avoid it at the cost of forcing all readers into the slow path.
    	 */
    	rcu_sync_enter_start(&cgroup_threadgroup_rwsem.rss);
    
    	get_user_ns(init_cgroup_ns.user_ns);
    
    	mutex_lock(&cgroup_mutex);
    
    	/*
    	 * Add init_css_set to the hash table so that dfl_root can link to
    	 * it during init.
    	 */
    	hash_add(css_set_table, &init_css_set.hlist,
    		 css_set_hash(init_css_set.subsys));
    
    	BUG_ON(cgroup_setup_root(&cgrp_dfl_root, 0, 0));
    
    	mutex_unlock(&cgroup_mutex);
    
    	for_each_subsys(ss, ssid) {
    		if (ss->early_init) {
    			struct cgroup_subsys_state *css =
    				init_css_set.subsys[ss->id];
    
    			css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2,
    						   GFP_KERNEL);
    			BUG_ON(css->id < 0);
    		} else {
    			cgroup_init_subsys(ss, false);
    		}
    
    		list_add_tail(&init_css_set.e_cset_node[ssid],
    			      &cgrp_dfl_root.cgrp.e_csets[ssid]);
    
    		/*
    		 * Setting dfl_root subsys_mask needs to consider the
    		 * disabled flag and cftype registration needs kmalloc,
    		 * both of which aren't available during early_init.
    		 */
    		if (cgroup_disable_mask & (1 << ssid)) {
    			static_branch_disable(cgroup_subsys_enabled_key[ssid]);
    			printk(KERN_INFO "Disabling %s control group subsystem\n",
    			       ss->name);
    			continue;
    		}
    
    		if (cgroup1_ssid_disabled(ssid))
    			printk(KERN_INFO "Disabling %s control group subsystem in v1 mounts\n",
    			       ss->name);
    
    		cgrp_dfl_root.subsys_mask |= 1 << ss->id;
    
    		if (ss->implicit_on_dfl)
    			cgrp_dfl_implicit_ss_mask |= 1 << ss->id;
    		else if (!ss->dfl_cftypes)
    			cgrp_dfl_inhibit_ss_mask |= 1 << ss->id;
    
    		if (ss->dfl_cftypes == ss->legacy_cftypes) {
    			WARN_ON(cgroup_add_cftypes(ss, ss->dfl_cftypes));
    		} else {
    			WARN_ON(cgroup_add_dfl_cftypes(ss, ss->dfl_cftypes));
    			WARN_ON(cgroup_add_legacy_cftypes(ss, ss->legacy_cftypes));
    		}
    
    		if (ss->bind)
    			ss->bind(init_css_set.subsys[ssid]);
    
    		mutex_lock(&cgroup_mutex);
    		css_populate_dir(init_css_set.subsys[ssid]);
    		mutex_unlock(&cgroup_mutex);
    	}
    
    	/* init_css_set.subsys[] has been updated, re-hash */
    	hash_del(&init_css_set.hlist);
    	hash_add(css_set_table, &init_css_set.hlist,
    		 css_set_hash(init_css_set.subsys));
    
    	WARN_ON(sysfs_create_mount_point(fs_kobj, "cgroup"));
    	WARN_ON(register_filesystem(&cgroup_fs_type));
    	WARN_ON(register_filesystem(&cgroup2_fs_type));
    	WARN_ON(!proc_create("cgroups", 0, NULL, &proc_cgroupstats_operations));
    
    	return 0;
    }
    
    static int __init cgroup_wq_init(void)
    {
    	/*
    	 * There isn't much point in executing destruction path in
    	 * parallel.  Good chunk is serialized with cgroup_mutex anyway.
    	 * Use 1 for @max_active.
    	 *
    	 * We would prefer to do this in cgroup_init() above, but that
    	 * is called before init_workqueues(): so leave this until after.
    	 */
    	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
    	BUG_ON(!cgroup_destroy_wq);
    	return 0;
    }
    core_initcall(cgroup_wq_init);
    
    /*
     * proc_cgroup_show()
     *  - Print task's cgroup paths into seq_file, one line for each hierarchy
     *  - Used for /proc/<pid>/cgroup.
     */
    int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
    		     struct pid *pid, struct task_struct *tsk)
    {
    	char *buf;
    	int retval;
    	struct cgroup_root *root;
    
    	retval = -ENOMEM;
    	buf = kmalloc(PATH_MAX, GFP_KERNEL);
    	if (!buf)
    		goto out;
    
    	mutex_lock(&cgroup_mutex);
    	spin_lock_irq(&css_set_lock);
    
    	for_each_root(root) {
    		struct cgroup_subsys *ss;
    		struct cgroup *cgrp;
    		int ssid, count = 0;
    
    		if (root == &cgrp_dfl_root && !cgrp_dfl_visible)
    			continue;
    
    		seq_printf(m, "%d:", root->hierarchy_id);
    		if (root != &cgrp_dfl_root)
    			for_each_subsys(ss, ssid)
    				if (root->subsys_mask & (1 << ssid))
    					seq_printf(m, "%s%s", count++ ? "," : "",
    						   ss->legacy_name);
    		if (strlen(root->name))
    			seq_printf(m, "%sname=%s", count ? "," : "",
    				   root->name);
    		seq_putc(m, ':');
    
    		cgrp = task_cgroup_from_root(tsk, root);
    
    		/*
    		 * On traditional hierarchies, all zombie tasks show up as
    		 * belonging to the root cgroup.  On the default hierarchy,
    		 * while a zombie doesn't show up in "cgroup.procs" and
    		 * thus can't be migrated, its /proc/PID/cgroup keeps
    		 * reporting the cgroup it belonged to before exiting.  If
    		 * the cgroup is removed before the zombie is reaped,
    		 * " (deleted)" is appended to the cgroup path.
    		 */
    		if (cgroup_on_dfl(cgrp) || !(tsk->flags & PF_EXITING)) {
    			retval = cgroup_path_ns_locked(cgrp, buf, PATH_MAX,
    						current->nsproxy->cgroup_ns);
    			if (retval >= PATH_MAX)
    				retval = -ENAMETOOLONG;
    			if (retval < 0)
    				goto out_unlock;
    
    			seq_puts(m, buf);
    		} else {
    			seq_puts(m, "/");
    		}
    
    		if (cgroup_on_dfl(cgrp) && cgroup_is_dead(cgrp))
    			seq_puts(m, " (deleted)\n");
    		else
    			seq_putc(m, '\n');
    	}
    
    	retval = 0;
    out_unlock:
    	spin_unlock_irq(&css_set_lock);
    	mutex_unlock(&cgroup_mutex);
    	kfree(buf);
    out:
    	return retval;
    }
    
    /**
     * cgroup_fork - initialize cgroup related fields during copy_process()
     * @child: pointer to task_struct of forking parent process.
     *
     * A task is associated with the init_css_set until cgroup_post_fork()
     * attaches it to the parent's css_set.  Empty cg_list indicates that
     * @child isn't holding reference to its css_set.
     */
    void cgroup_fork(struct task_struct *child)
    {
    	RCU_INIT_POINTER(child->cgroups, &init_css_set);
    	INIT_LIST_HEAD(&child->cg_list);
    }
    
    /**
     * cgroup_can_fork - called on a new task before the process is exposed
     * @child: the task in question.
     *
     * This calls the subsystem can_fork() callbacks. If the can_fork() callback
     * returns an error, the fork aborts with that error code. This allows for
     * a cgroup subsystem to conditionally allow or deny new forks.
     */
    int cgroup_can_fork(struct task_struct *child)
    {
    	struct cgroup_subsys *ss;
    	int i, j, ret;
    
    	do_each_subsys_mask(ss, i, have_canfork_callback) {
    		ret = ss->can_fork(child);
    		if (ret)
    			goto out_revert;
    	} while_each_subsys_mask();
    
    	return 0;
    
    out_revert:
    	for_each_subsys(ss, j) {
    		if (j >= i)
    			break;
    		if (ss->cancel_fork)
    			ss->cancel_fork(child);
    	}
    
    	return ret;
    }
    
    /**
     * cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
     * @child: the task in question
     *
     * This calls the cancel_fork() callbacks if a fork failed *after*
     * cgroup_can_fork() succeded.
     */
    void cgroup_cancel_fork(struct task_struct *child)
    {
    	struct cgroup_subsys *ss;
    	int i;
    
    	for_each_subsys(ss, i)
    		if (ss->cancel_fork)
    			ss->cancel_fork(child);
    }
    
    /**
     * cgroup_post_fork - called on a new task after adding it to the task list
     * @child: the task in question
     *
     * Adds the task to the list running through its css_set if necessary and
     * call the subsystem fork() callbacks.  Has to be after the task is
     * visible on the task list in case we race with the first call to
     * cgroup_task_iter_start() - to guarantee that the new task ends up on its
     * list.
     */
    void cgroup_post_fork(struct task_struct *child)
    {
    	struct cgroup_subsys *ss;
    	int i;
    
    	/*
    	 * This may race against cgroup_enable_task_cg_lists().  As that
    	 * function sets use_task_css_set_links before grabbing
    	 * tasklist_lock and we just went through tasklist_lock to add
    	 * @child, it's guaranteed that either we see the set
    	 * use_task_css_set_links or cgroup_enable_task_cg_lists() sees
    	 * @child during its iteration.
    	 *
    	 * If we won the race, @child is associated with %current's
    	 * css_set.  Grabbing css_set_lock guarantees both that the
    	 * association is stable, and, on completion of the parent's
    	 * migration, @child is visible in the source of migration or
    	 * already in the destination cgroup.  This guarantee is necessary
    	 * when implementing operations which need to migrate all tasks of
    	 * a cgroup to another.
    	 *
    	 * Note that if we lose to cgroup_enable_task_cg_lists(), @child
    	 * will remain in init_css_set.  This is safe because all tasks are
    	 * in the init_css_set before cg_links is enabled and there's no
    	 * operation which transfers all tasks out of init_css_set.
    	 */
    	if (use_task_css_set_links) {
    		struct css_set *cset;
    
    		spin_lock_irq(&css_set_lock);
    		cset = task_css_set(current);
    		if (list_empty(&child->cg_list)) {
    			get_css_set(cset);
    			cset->nr_tasks++;
    			css_set_move_task(child, NULL, cset, false);
    		}
    		spin_unlock_irq(&css_set_lock);
    	}
    
    	/*
    	 * Call ss->fork().  This must happen after @child is linked on
    	 * css_set; otherwise, @child might change state between ->fork()
    	 * and addition to css_set.
    	 */
    	do_each_subsys_mask(ss, i, have_fork_callback) {
    		ss->fork(child);
    	} while_each_subsys_mask();
    }
    
    /**
     * cgroup_exit - detach cgroup from exiting task
     * @tsk: pointer to task_struct of exiting process
     *
     * Description: Detach cgroup from @tsk and release it.
     *
     * Note that cgroups marked notify_on_release force every task in
     * them to take the global cgroup_mutex mutex when exiting.
     * This could impact scaling on very large systems.  Be reluctant to
     * use notify_on_release cgroups where very high task exit scaling
     * is required on large systems.
     *
     * We set the exiting tasks cgroup to the root cgroup (top_cgroup).  We
     * call cgroup_exit() while the task is still competent to handle
     * notify_on_release(), then leave the task attached to the root cgroup in
     * each hierarchy for the remainder of its exit.  No need to bother with
     * init_css_set refcnting.  init_css_set never goes away and we can't race
     * with migration path - PF_EXITING is visible to migration path.
     */
    void cgroup_exit(struct task_struct *tsk)
    {
    	struct cgroup_subsys *ss;
    	struct css_set *cset;
    	int i;
    
    	/*
    	 * Unlink from @tsk from its css_set.  As migration path can't race
    	 * with us, we can check css_set and cg_list without synchronization.
    	 */
    	cset = task_css_set(tsk);
    
    	if (!list_empty(&tsk->cg_list)) {
    		spin_lock_irq(&css_set_lock);
    		css_set_move_task(tsk, cset, NULL, false);
    		cset->nr_tasks--;
    		spin_unlock_irq(&css_set_lock);
    	} else {
    		get_css_set(cset);
    	}
    
    	/* see cgroup_post_fork() for details */
    	do_each_subsys_mask(ss, i, have_exit_callback) {
    		ss->exit(tsk);
    	} while_each_subsys_mask();
    }
    
    void cgroup_free(struct task_struct *task)
    {
    	struct css_set *cset = task_css_set(task);
    	struct cgroup_subsys *ss;
    	int ssid;
    
    	do_each_subsys_mask(ss, ssid, have_free_callback) {
    		ss->free(task);
    	} while_each_subsys_mask();
    
    	put_css_set(cset);
    }
    
    static int __init cgroup_disable(char *str)
    {
    	struct cgroup_subsys *ss;
    	char *token;
    	int i;
    
    	while ((token = strsep(&str, ",")) != NULL) {
    		if (!*token)
    			continue;
    
    		for_each_subsys(ss, i) {
    			if (strcmp(token, ss->name) &&
    			    strcmp(token, ss->legacy_name))
    				continue;
    			cgroup_disable_mask |= 1 << i;
    		}
    	}
    	return 1;
    }
    __setup("cgroup_disable=", cgroup_disable);
    
    /**
     * css_tryget_online_from_dir - get corresponding css from a cgroup dentry
     * @dentry: directory dentry of interest
     * @ss: subsystem of interest
     *
     * If @dentry is a directory for a cgroup which has @ss enabled on it, try
     * to get the corresponding css and return it.  If such css doesn't exist
     * or can't be pinned, an ERR_PTR value is returned.
     */
    struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
    						       struct cgroup_subsys *ss)
    {
    	struct kernfs_node *kn = kernfs_node_from_dentry(dentry);
    	struct file_system_type *s_type = dentry->d_sb->s_type;
    	struct cgroup_subsys_state *css = NULL;
    	struct cgroup *cgrp;
    
    	/* is @dentry a cgroup dir? */
    	if ((s_type != &cgroup_fs_type && s_type != &cgroup2_fs_type) ||
    	    !kn || kernfs_type(kn) != KERNFS_DIR)
    		return ERR_PTR(-EBADF);
    
    	rcu_read_lock();
    
    	/*
    	 * This path doesn't originate from kernfs and @kn could already
    	 * have been or be removed at any point.  @kn->priv is RCU
    	 * protected for this access.  See css_release_work_fn() for details.
    	 */
    	cgrp = rcu_dereference(*(void __rcu __force **)&kn->priv);
    	if (cgrp)
    		css = cgroup_css(cgrp, ss);
    
    	if (!css || !css_tryget_online(css))
    		css = ERR_PTR(-ENOENT);
    
    	rcu_read_unlock();
    	return css;
    }
    
    /**
     * css_from_id - lookup css by id
     * @id: the cgroup id
     * @ss: cgroup subsys to be looked into
     *
     * Returns the css if there's valid one with @id, otherwise returns NULL.
     * Should be called under rcu_read_lock().
     */
    struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
    {
    	WARN_ON_ONCE(!rcu_read_lock_held());
    	return idr_find(&ss->css_idr, id);
    }
    
    /**
     * cgroup_get_from_path - lookup and get a cgroup from its default hierarchy path
     * @path: path on the default hierarchy
     *
     * Find the cgroup at @path on the default hierarchy, increment its
     * reference count and return it.  Returns pointer to the found cgroup on
     * success, ERR_PTR(-ENOENT) if @path doens't exist and ERR_PTR(-ENOTDIR)
     * if @path points to a non-directory.
     */
    struct cgroup *cgroup_get_from_path(const char *path)
    {
    	struct kernfs_node *kn;
    	struct cgroup *cgrp;
    
    	mutex_lock(&cgroup_mutex);
    
    	kn = kernfs_walk_and_get(cgrp_dfl_root.cgrp.kn, path);
    	if (kn) {
    		if (kernfs_type(kn) == KERNFS_DIR) {
    			cgrp = kn->priv;
    			cgroup_get_live(cgrp);
    		} else {
    			cgrp = ERR_PTR(-ENOTDIR);
    		}
    		kernfs_put(kn);
    	} else {
    		cgrp = ERR_PTR(-ENOENT);
    	}
    
    	mutex_unlock(&cgroup_mutex);
    	return cgrp;
    }
    EXPORT_SYMBOL_GPL(cgroup_get_from_path);
    
    /**
     * cgroup_get_from_fd - get a cgroup pointer from a fd
     * @fd: fd obtained by open(cgroup2_dir)
     *
     * Find the cgroup from a fd which should be obtained
     * by opening a cgroup directory.  Returns a pointer to the
     * cgroup on success. ERR_PTR is returned if the cgroup
     * cannot be found.
     */
    struct cgroup *cgroup_get_from_fd(int fd)
    {
    	struct cgroup_subsys_state *css;
    	struct cgroup *cgrp;
    	struct file *f;
    
    	f = fget_raw(fd);
    	if (!f)
    		return ERR_PTR(-EBADF);
    
    	css = css_tryget_online_from_dir(f->f_path.dentry, NULL);
    	fput(f);
    	if (IS_ERR(css))
    		return ERR_CAST(css);
    
    	cgrp = css->cgroup;
    	if (!cgroup_on_dfl(cgrp)) {
    		cgroup_put(cgrp);
    		return ERR_PTR(-EBADF);
    	}
    
    	return cgrp;
    }
    EXPORT_SYMBOL_GPL(cgroup_get_from_fd);
    
    /*
     * sock->sk_cgrp_data handling.  For more info, see sock_cgroup_data
     * definition in cgroup-defs.h.
     */
    #ifdef CONFIG_SOCK_CGROUP_DATA
    
    #if defined(CONFIG_CGROUP_NET_PRIO) || defined(CONFIG_CGROUP_NET_CLASSID)
    
    DEFINE_SPINLOCK(cgroup_sk_update_lock);
    static bool cgroup_sk_alloc_disabled __read_mostly;
    
    void cgroup_sk_alloc_disable(void)
    {
    	if (cgroup_sk_alloc_disabled)
    		return;
    	pr_info("cgroup: disabling cgroup2 socket matching due to net_prio or net_cls activation\n");
    	cgroup_sk_alloc_disabled = true;
    }
    
    #else
    
    #define cgroup_sk_alloc_disabled	false
    
    #endif
    
    void cgroup_sk_alloc(struct sock_cgroup_data *skcd)
    {
    	if (cgroup_sk_alloc_disabled)
    		return;
    
    	/* Socket clone path */
    	if (skcd->val) {
    		/*
    		 * We might be cloning a socket which is left in an empty
    		 * cgroup and the cgroup might have already been rmdir'd.
    		 * Don't use cgroup_get_live().
    		 */
    		cgroup_get(sock_cgroup_ptr(skcd));
    		return;
    	}
    
    	rcu_read_lock();
    
    	while (true) {
    		struct css_set *cset;
    
    		cset = task_css_set(current);
    		if (likely(cgroup_tryget(cset->dfl_cgrp))) {
    			skcd->val = (unsigned long)cset->dfl_cgrp;
    			break;
    		}
    		cpu_relax();
    	}
    
    	rcu_read_unlock();
    }
    
    void cgroup_sk_free(struct sock_cgroup_data *skcd)
    {
    	cgroup_put(sock_cgroup_ptr(skcd));
    }
    
    #endif	/* CONFIG_SOCK_CGROUP_DATA */
    
    #ifdef CONFIG_CGROUP_BPF
    int cgroup_bpf_update(struct cgroup *cgrp, struct bpf_prog *prog,
    		      enum bpf_attach_type type, bool overridable)
    {
    	struct cgroup *parent = cgroup_parent(cgrp);
    	int ret;
    
    	mutex_lock(&cgroup_mutex);
    	ret = __cgroup_bpf_update(cgrp, parent, prog, type, overridable);
    	mutex_unlock(&cgroup_mutex);
    	return ret;
    }
    #endif /* CONFIG_CGROUP_BPF */