workqueue: add missing smp_wmb() in process_one_work()

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@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 70f95ab28f3dbc91ba119c0c4418311014b94b71..5c26d36146b7cdaffd4529973413ceb2fe4bb463 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1997,7 +1997,9 @@ __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);