Commit 4ce48b37 authored Jul 02, 2010 by Tejun Heo

workqueue: fix race condition in flush_workqueue()



When one flusher is cascading to the next flusher, it first sets
wq->first_flusher to the next one and sets up the next flush cycle.
If there's nothing to do for the next cycle, it clears
wq->flush_flusher and proceeds to the one after that.

If the woken up flusher checks wq->first_flusher before it gets
cleared, it will incorrectly assume the role of the first flusher,
which triggers BUG_ON() sanity check.

Fix it by checking wq->first_flusher again after grabbing the mutex.

Signed-off-by: Tejun Heo <tj@kernel.org>

parent cb444766

kernel/workqueue.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -2138,6 +2138,10 @@ void flush_workqueue(struct workqueue_struct *wq)

		mutex_lock(&wq->flush_mutex);

		/* we might have raced, check again with mutex held */
		if (wq->first_flusher != &this_flusher)
		goto out_unlock;

		wq->first_flusher = NULL;

		BUG_ON(!list_empty(&this_flusher.list));