Commit 52fa5bc5 authored by Boqun Feng's avatar Boqun Feng Committed by Ingo Molnar
Browse files

locking/lockdep: Explicitly initialize wq_barrier::done::map

With the new lockdep crossrelease feature, which checks completions usage,
a false positive is reported in the workqueue code:

> Worker A : acquired of -> wait for cpu_hotplug_lock to be released
> Task   B : acquired of cpu_hotplug_lock -> wait for lock#3 to be released
> Task   C : acquired of lock#3 -> wait for completion of barr->done
> (Task C is in lru_add_drain_all_cpuslocked())
> Worker D : wait for to be released -> will complete barr->done

Such a dead lock can not happen because Task C's barr->done and Worker D's
barr->done can not be the same instance.

The reason of this false positive is we initialize all wq_barrier::done
at insert_wq_barrier() via init_completion(), which makes them belong to
the same lock class, therefore, impossible circles are reported.

To fix this, explicitly initialize the lockdep map for wq_barrier::done
in insert_wq_barrier(), so that the lock class key of wq_barrier::done
is a subkey of the corresponding work_struct, as a result we won't build
a dependency between a wq_barrier with a unrelated work, and we can
differ wq barriers based on the related works, so the false positive
above is avoided.

Also define the empty lockdep_init_map_crosslock() for !CROSSRELEASE
to make the code simple and away from unnecessary #ifdefs.
Reported-by: default avatarIngo Molnar <>
Signed-off-by: default avatarBoqun Feng <>
Cc: Byungchul Park <>
Cc: Lai Jiangshan <>
Cc: Linus Torvalds <>
Cc: Peter Zijlstra <>
Cc: Tejun Heo <>
Cc: Thomas Gleixner <>

Signed-off-by: default avatarIngo Molnar <>
parent ea3f2c0f
......@@ -583,6 +583,7 @@ 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);
#define lockdep_init_map_crosslock(m, n, k, s) do {} while (0)
* To initialize a lockdep_map statically use this macro.
* Note that _name must not be NULL.
......@@ -2476,7 +2476,16 @@ static void insert_wq_barrier(struct pool_workqueue *pwq,
INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);
__set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
* Explicitly init the crosslock for wq_barrier::done, make its lock
* key a subkey of the corresponding work. As a result we won't
* build a dependency between wq_barrier::done and unrelated work.
lockdep_init_map_crosslock((struct lockdep_map *)&barr->,
target->lockdep_map.key, 1);
barr->task = current;
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