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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@chromium.org>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: stable@kernel.org
---
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

----- End forwarded message -----

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