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Subject[128/244] workqueue: lock cwq access in drain_workqueue
3.0-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Thomas Tuttle <ttuttle@chromium.org>

commit fa2563e41c3d6d6e8af437643981ed28ae0cb56d upstream.

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@chromium.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/workqueue.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3026,8 +3026,13 @@ 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 ||



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