[ 019/120] workqueue: add missing smp_wmb() in process_one_work()

[Greg Kroah-Hartman]

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Tejun Heo <tj@xxxxxxxxxx>

commit 959d1af8cffc8fd38ed53e8be1cf4ab8782f9c00 upstream.

WORK_STRUCT_PENDING is used to claim ownership of a work item and
process_one_work() releases it before starting execution.  When
someone else grabs PENDING, all pre-release updates to the work item
should be visible and all updates made by the new owner should happen
afterwards.

Grabbing PENDING uses test_and_set_bit() and thus has a full barrier;
however, clearing doesn't have a matching wmb.  Given the preceding
spin_unlock and use of clear_bit, I don't believe this can be a
problem on an actual machine and there hasn't been any related report
but it still is theretically possible for clear_pending to permeate
upwards and happen before work->entry update.

Add an explicit smp_wmb() before work_clear_pending().

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/workqueue.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1864,7 +1864,9 @@ __acquires(&gcwq->lock)
 
 	spin_unlock_irq(&gcwq->lock);
 
+	smp_wmb();	/* paired with test_and_set_bit(PENDING) */
 	work_clear_pending(work);
+
 	lock_map_acquire_read(&cwq->wq->lockdep_map);
 	lock_map_acquire(&lockdep_map);
 	trace_workqueue_execute_start(work);


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