linux-kernel - [PATCH v3] workqueue: lock cwq access in drain

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Message-ID: <20110911032641.GA22325@google.com>
Date:	Sat, 10 Sep 2011 23:26:41 -0400
From:	Thomas Tuttle <ttuttle@...omium.org>
To:	Tejun Heo <tj@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: [PATCH v3] workqueue: lock cwq access in drain_workqueue

Take cwq->gcwq->lock to avoid racing between drain_workqueue checking
to make sure the workqueues are empty and cwq_dec_nr_in_flight
decrementing and then incrementing nr_active when it activates a
delayed work.

We discovered this when a corner case in one of our drivers resulted in
us trying to destroy a workqueue in which the remaining work would
always requeue itself again in the same workqueue.  We would hit this
race condition and trip the BUG_ON on workqueue.c:3080.

Signed-off-by: Thomas Tuttle <ttuttle@...omium.org>
---
Renamed "cwq_flushed" to "drained" as requested and rebased against
current HEAD (d0a77454c70d0449a5f87087deb8f0cb15145e90).

 kernel/workqueue.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 25fb1b0..1783aab 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2412,8 +2412,13 @@ reflush:
 
 	for_each_cwq_cpu(cpu, wq) {
 		struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
+		bool drained;
 
-		if (!cwq->nr_active && list_empty(&cwq->delayed_works))
+		spin_lock_irq(&cwq->gcwq->lock);
+		drained = !cwq->nr_active && list_empty(&cwq->delayed_works);
+		spin_unlock_irq(&cwq->gcwq->lock);
+
+		if (drained)
 			continue;
 
 		if (++flush_cnt == 10 ||
-- 
1.7.3.1

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