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    SubjectRe: [PATCH 1/2] workqueue: Catch more locking problems with flush_work()
    On 04/19/12 08:28, Tejun Heo wrote:
    > On Wed, Apr 18, 2012 at 08:25:57PM -0700, Stephen Boyd wrote:
    >> @@ -2513,8 +2513,11 @@ bool flush_work(struct work_struct *work)
    >> wait_for_completion(&barr.done);
    >> destroy_work_on_stack(&barr.work);
    >> return true;
    >> - } else
    >> + } else {
    >> + lock_map_acquire(&work->lockdep_map);
    >> + lock_map_release(&work->lockdep_map);
    >> return false;
    > We don't have this annotation when start_flush_work() succeeds either,
    > right? IOW, would lockdep trigger when an actual deadlock happens?

    I believe it does although I haven't tested it.

    > If not, why not add the acquire/release() before flush_work() does
    > anything?
    >

    I was worried about causing false positive lockdep warnings in the case
    that start_flush_work() succeeds and returns true. In that case, lockdep
    is told about the cwq lockdep map:

    static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
    bool wait_executing)
    {

    .....

    if (cwq->wq->saved_max_active == 1 || cwq->wq->flags & WQ_RESCUER)
    lock_map_acquire(&cwq->wq->lockdep_map);
    else
    lock_map_acquire_read(&cwq->wq->lockdep_map);


    and so if we acquired the work->lockdep_map before the
    cwq->wq->lockdep_map we would get a warning about ABBA between these two
    lockdep maps. At least that is what I'm lead to believe when I look at
    what process_one_work() is doing. Please correct me if I'm wrong.

    --
    Sent by an employee of the Qualcomm Innovation Center, Inc.
    The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.



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