Commit a1d14934 authored Aug 23, 2017 by Peter Zijlstra Committed by Ingo Molnar Aug 25, 2017

workqueue/lockdep: 'Fix' flush_work() annotation



The flush_work() annotation as introduced by commit:

  e159489b ("workqueue: relax lockdep annotation on flush_work()")

hits on the lockdep problem with recursive read locks.

The situation as described is:

Work W1:                Work W2:        Task:

ARR(Q)                  ARR(Q)		flush_workqueue(Q)
A(W1)                   A(W2)             A(Q)
  flush_work(W2)			  R(Q)
    A(W2)
    R(W2)
    if (special)
      A(Q)
    else
      ARR(Q)
    R(Q)

where: A - acquire, ARR - acquire-read-recursive, R - release.

Where under 'special' conditions we want to trigger a lock recursion
deadlock, but otherwise allow the flush_work(). The allowing is done
by using recursive read locks (ARR), but lockdep is broken for
recursive stuff.

However, there appears to be no need to acquire the lock if we're not
'special', so if we remove the 'else' clause things become much
simpler and no longer need the recursion thing at all.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun 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: Ingo Molnar <mingo@kernel.org>

parent e9149858

kernel/workqueue.c

+11 −9

Original line number	Diff line number	Diff line
		@@ -2091,7 +2091,7 @@ __acquires(&pool->lock)

		spin_unlock_irq(&pool->lock);

		lock_map_acquire_read(&pwq->wq->lockdep_map);
		lock_map_acquire(&pwq->wq->lockdep_map);
		lock_map_acquire(&lockdep_map);
		crossrelease_hist_start(XHLOCK_PROC);
		trace_workqueue_execute_start(work);
		@@ -2826,16 +2826,18 @@ static bool start_flush_work(struct work_struct work, struct wq_barrier barr)
		spin_unlock_irq(&pool->lock);

		/*
		* If @max_active is 1 or rescuer is in use, flushing another work
		* item on the same workqueue may lead to deadlock. Make sure the
		* flusher is not running on the same workqueue by verifying write
		* access.
		* Force a lock recursion deadlock when using flush_work() inside a
		* single-threaded or rescuer equipped workqueue.
		*
		* For single threaded workqueues the deadlock happens when the work
		* is after the work issuing the flush_work(). For rescuer equipped
		* workqueues the deadlock happens when the rescuer stalls, blocking
		* forward progress.
		*/
		if (pwq->wq->saved_max_active == 1 \|\| pwq->wq->rescuer)
		if (pwq->wq->saved_max_active == 1 \|\| pwq->wq->rescuer) {
		lock_map_acquire(&pwq->wq->lockdep_map);
		else
		lock_map_acquire_read(&pwq->wq->lockdep_map);
		lock_map_release(&pwq->wq->lockdep_map);
		}

		return true;
		already_gone:

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