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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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