Commit 52fa5bc5 authored Aug 17, 2017 by Boqun Feng Committed by Ingo Molnar Aug 17, 2017

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 wfc.work -> 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 wfc.work 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: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170817094622.12915-1-boqun.feng@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent ea3f2c0f

include/linux/lockdep.h

+1 −0

Original line number	Original line	Diff line number	Diff line
	@@ -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_init_task(struct task_struct *task);
	extern void lockdep_free_task(struct task_struct *task);		extern void lockdep_free_task(struct task_struct *task);
	#else		#else
			#define lockdep_init_map_crosslock(m, n, k, s) do {} while (0)
	/*		/*
	* To initialize a lockdep_map statically use this macro.		* To initialize a lockdep_map statically use this macro.
	* Note that _name must not be NULL.		* Note that _name must not be NULL.

kernel/workqueue.c

+10 −1

Original line number	Original line	Diff line number	Diff line
	@@ -2476,7 +2476,16 @@ static void insert_wq_barrier(struct pool_workqueue *pwq,
	*/		*/
	INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);		INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);
	__set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));		__set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
	init_completion(&barr->done);
			/*
			* 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->done.map,
			"(complete)wq_barr::done",
			target->lockdep_map.key, 1);
			__init_completion(&barr->done);
	barr->task = current;		barr->task = current;

	/*		/*

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