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workqueue: fix race condition in flush_workqueue()

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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>
This commit is contained in:
Tejun Heo 2010-07-02 10:03:51 +02:00
parent cb44476699
commit 4ce48b37bf
1 changed files with 4 additions and 0 deletions
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4
kernel/workqueue.c
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@ -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));