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    SubjectRe: [PATCHv2] backing-dev: fix wakeup timer races with bdi_unregister()
    From
    2012/1/20 Rabin Vincent <rabin@rab.in>:
    > On Fri, Jan 20, 2012 at 05:16, Namjae Jeon <linkinjeon@gmail.com> wrote:
    >>>                bdi_debug_unregister(bdi);
    >>> -               device_unregister(bdi->dev);
    >>> +
    >>> +               spin_lock_bh(&bdi->wb_lock);
    >>>                bdi->dev = NULL;
    >>> +               spin_unlock_bh(&bdi->wb_lock);
    >> Hi.
    >> Would you explain me why you add spinlock in here ?
    >
    > wakeup_timer_fn() does the following, where the
    > trace_writeback_wake_forker_thread() also accesses bdi->dev.
    > It does this under the wb_lock:
    >
    >        } else if (bdi->dev) {
    >                /*
    >                 * When bdi tasks are inactive for long time, they are killed.
    >                 * In this case we have to wake-up the forker thread which
    >                 * should create and run the bdi thread.
    >                 */
    >                trace_writeback_wake_forker_thread(bdi);
    >
    > If we don't have the lock above, the bdi->dev could potentially be
    > cleared after the check but before the tracepoint is hit, leading to a
    > NULL pointer dereference.
    Is there no possibility trace_writeback_wake_forker_thread is called
    after spin_unlock of bdi->de= null ?
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