workqueue: lock cwq access in drain_workqueue

BugLink: http://bugs.launchpad.net/bugs/868628

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>

1 files changed, 6 insertions, 1 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 0400553f0d0..aec02b6a1c4 100644
--- 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 ||