linux-kernel - [ 004/147] workqueue: add missing smp_wmb() in process_one

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Date:	Sun, 14 Oct 2012 15:35:37 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>
Subject: [ 004/147] workqueue: add missing smp_wmb() in process_one_work()

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

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

From: Tejun Heo <tj@...nel.org>

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@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/workqueue.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 70f95ab..5c26d36 100644
--- 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);

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