[PATCH] workqueue: lock cwq access in drain_workqueue

[Thomas Tuttle <ttuttle@chromium.org>]

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.

Patch is against HEAD as of Fri Sep  9 15:16:09 UTC 2011
(e4e436e0bd480668834fe6849a52c5397b7be4fb).

Signed-off-by: Thomas Tuttle <ttuttle@chromium.org>
---
 kernel/workqueue.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 25fb1b0..d610ced 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2412,8 +2412,14 @@ reflush:
 
 	for_each_cwq_cpu(cpu, wq) {
 		struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
+		int cwq_flushed;
 
-		if (!cwq->nr_active && list_empty(&cwq->delayed_works))
+		spin_lock_irq(&cwq->gcwq->lock);
+		cwq_flushed = !cwq->nr_active
+			      && list_empty(&cwq->delayed_works);
+		spin_unlock_irq(&cwq->gcwq->lock);
+
+		if (cwq_flushed)
 			continue;
 
 		if (++flush_cnt == 10 ||
-- 
1.7.3.1