linux-kernel - [PATCH 03/14] workqueue: add missing smp_wmb() in process_one

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Date:	Fri,  3 Aug 2012 10:43:48 -0700
From:	Tejun Heo <tj@...nel.org>
To:	linux-kernel@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	padovan@...fusion.mobi, marcel@...tmann.org, peterz@...radead.org,
	mingo@...hat.com, davem@...emloft.net, dougthompson@...ssion.com,
	ibm-acpi@....eng.br, cbou@...l.ru, rui.zhang@...el.com,
	tomi.valkeinen@...com, Tejun Heo <tj@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>, stable@...r.kernel.org
Subject: [PATCH 03/14] 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@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: stable@...r.kernel.org
---
 kernel/workqueue.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

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);
-- 
1.7.7.3

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