Commit e6f3faa7 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
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locking/lockdep: Fix workqueue crossrelease annotation



The new completion/crossrelease annotations interact unfavourable with
the extant flush_work()/flush_workqueue() annotations.

The problem is that when a single work class does:

  wait_for_completion(&C)

and

  complete(&C)

in different executions, we'll build dependencies like:

  lock_map_acquire(W)
  complete_acquire(C)

and

  lock_map_acquire(W)
  complete_release(C)

which results in the dependency chain: W->C->W, which lockdep thinks
spells deadlock, even though there is no deadlock potential since
works are ran concurrently.

One possibility would be to change the work 'lock' to recursive-read,
but that would mean hitting a lockdep limitation on recursive locks.
Also, unconditinoally switching to recursive-read here would fail to
detect the actual deadlock on single-threaded workqueues, which do
have a problem with this.

For now, forcefully disregard these locks for crossrelease.
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boqun.feng@gmail.com
Cc: byungchul.park@lge.com
Cc: david@fromorbit.com
Cc: johannes@sipsolutions.net
Cc: oleg@redhat.com
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent a1d14934
......@@ -26,7 +26,7 @@
# define trace_hardirq_enter() \
do { \
current->hardirq_context++; \
crossrelease_hist_start(XHLOCK_HARD); \
crossrelease_hist_start(XHLOCK_HARD, 0);\
} while (0)
# define trace_hardirq_exit() \
do { \
......@@ -36,7 +36,7 @@ do { \
# define lockdep_softirq_enter() \
do { \
current->softirq_context++; \
crossrelease_hist_start(XHLOCK_SOFT); \
crossrelease_hist_start(XHLOCK_SOFT, 0);\
} while (0)
# define lockdep_softirq_exit() \
do { \
......
......@@ -18,6 +18,8 @@ extern int lock_stat;
#define MAX_LOCKDEP_SUBCLASSES 8UL
#include <linux/types.h>
#ifdef CONFIG_LOCKDEP
#include <linux/linkage.h>
......@@ -578,11 +580,11 @@ extern void lock_commit_crosslock(struct lockdep_map *lock);
#define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
{ .name = (_name), .key = (void *)(_key), .cross = 0, }
extern void crossrelease_hist_start(enum xhlock_context_t c);
extern void crossrelease_hist_start(enum xhlock_context_t c, bool force);
extern void crossrelease_hist_end(enum xhlock_context_t c);
extern void lockdep_init_task(struct task_struct *task);
extern void lockdep_free_task(struct task_struct *task);
#else
#else /* !CROSSRELEASE */
#define lockdep_init_map_crosslock(m, n, k, s) do {} while (0)
/*
* To initialize a lockdep_map statically use this macro.
......@@ -591,11 +593,11 @@ extern void lockdep_free_task(struct task_struct *task);
#define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
{ .name = (_name), .key = (void *)(_key), }
static inline void crossrelease_hist_start(enum xhlock_context_t c) {}
static inline void crossrelease_hist_start(enum xhlock_context_t c, bool force) {}
static inline void crossrelease_hist_end(enum xhlock_context_t c) {}
static inline void lockdep_init_task(struct task_struct *task) {}
static inline void lockdep_free_task(struct task_struct *task) {}
#endif
#endif /* CROSSRELEASE */
#ifdef CONFIG_LOCK_STAT
......
......@@ -4629,7 +4629,7 @@ asmlinkage __visible void lockdep_sys_exit(void)
* the index to point to the last entry, which is already invalid.
*/
crossrelease_hist_end(XHLOCK_PROC);
crossrelease_hist_start(XHLOCK_PROC);
crossrelease_hist_start(XHLOCK_PROC, false);
}
void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
......@@ -4725,25 +4725,25 @@ static inline void invalidate_xhlock(struct hist_lock *xhlock)
/*
* Lock history stacks; we have 3 nested lock history stacks:
*
* Hard IRQ
* Soft IRQ
* History / Task
* HARD(IRQ)
* SOFT(IRQ)
* PROC(ess)
*
* The thing is that once we complete a (Hard/Soft) IRQ the future task locks
* should not depend on any of the locks observed while running the IRQ.
* The thing is that once we complete a HARD/SOFT IRQ the future task locks
* should not depend on any of the locks observed while running the IRQ. So
* what we do is rewind the history buffer and erase all our knowledge of that
* temporal event.
*
* So what we do is rewind the history buffer and erase all our knowledge of
* that temporal event.
*/
/*
* We need this to annotate lock history boundaries. Take for instance
* workqueues; each work is independent of the last. The completion of a future
* work does not depend on the completion of a past work (in general).
* Therefore we must not carry that (lock) dependency across works.
* The PROCess one is special though; it is used to annotate independence
* inside a task.
*
* Take for instance workqueues; each work is independent of the last. The
* completion of a future work does not depend on the completion of a past work
* (in general). Therefore we must not carry that (lock) dependency across
* works.
*
* This is true for many things; pretty much all kthreads fall into this
* pattern, where they have an 'idle' state and future completions do not
* pattern, where they have an invariant state and future completions do not
* depend on past completions. Its just that since they all have the 'same'
* form -- the kthread does the same over and over -- it doesn't typically
* matter.
......@@ -4751,15 +4751,31 @@ static inline void invalidate_xhlock(struct hist_lock *xhlock)
* The same is true for system-calls, once a system call is completed (we've
* returned to userspace) the next system call does not depend on the lock
* history of the previous system call.
*
* They key property for independence, this invariant state, is that it must be
* a point where we hold no locks and have no history. Because if we were to
* hold locks, the restore at _end() would not necessarily recover it's history
* entry. Similarly, independence per-definition means it does not depend on
* prior state.
*/
void crossrelease_hist_start(enum xhlock_context_t c)
void crossrelease_hist_start(enum xhlock_context_t c, bool force)
{
struct task_struct *cur = current;
if (cur->xhlocks) {
cur->xhlock_idx_hist[c] = cur->xhlock_idx;
cur->hist_id_save[c] = cur->hist_id;
if (!cur->xhlocks)
return;
/*
* We call this at an invariant point, no current state, no history.
*/
if (c == XHLOCK_PROC) {
/* verified the former, ensure the latter */
WARN_ON_ONCE(!force && cur->lockdep_depth);
invalidate_xhlock(&xhlock(cur->xhlock_idx));
}
cur->xhlock_idx_hist[c] = cur->xhlock_idx;
cur->hist_id_save[c] = cur->hist_id;
}
void crossrelease_hist_end(enum xhlock_context_t c)
......
......@@ -2093,7 +2093,28 @@ __acquires(&pool->lock)
lock_map_acquire(&pwq->wq->lockdep_map);
lock_map_acquire(&lockdep_map);
crossrelease_hist_start(XHLOCK_PROC);
/*
* Strictly speaking we should do start(PROC) without holding any
* locks, that is, before these two lock_map_acquire()'s.
*
* However, that would result in:
*
* A(W1)
* WFC(C)
* A(W1)
* C(C)
*
* Which would create W1->C->W1 dependencies, even though there is no
* actual deadlock possible. There are two solutions, using a
* read-recursive acquire on the work(queue) 'locks', but this will then
* hit the lockdep limitation on recursive locks, or simly discard
* these locks.
*
* AFAICT there is no possible deadlock scenario between the
* flush_work() and complete() primitives (except for single-threaded
* workqueues), so hiding them isn't a problem.
*/
crossrelease_hist_start(XHLOCK_PROC, true);
trace_workqueue_execute_start(work);
worker->current_func(work);
/*
......
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