Re: [PATCHv2] backing-dev: fix wakeup timer races with bdi_unregister()

[Namjae Jeon]

2012/1/20 Rabin Vincent <rabin@xxxxxx>:
> On Fri, Jan 20, 2012 at 05:16, Namjae Jeon <linkinjeon@xxxxxxxxx> wrote:
>>> Â Â Â Â Â Â Â Âbdi_debug_unregister(bdi);
>>> - Â Â Â Â Â Â Â device_unregister(bdi->dev);
>>> +
>>> + Â Â Â Â Â Â Â spin_lock_bh(&bdi->wb_lock);
>>> Â Â Â Â Â Â Â Âbdi->dev = NULL;
>>> + Â Â Â Â Â Â Â 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:
>
> Â Â Â Â} else if (bdi->dev) {
> Â Â Â Â Â Â Â Â/*
> Â Â Â Â Â Â Â Â * When bdi tasks are inactive for long time, they are killed.
> Â Â Â Â Â Â Â Â * In this case we have to wake-up the forker thread which
> Â Â Â Â Â Â Â Â * should create and run the bdi thread.
> Â Â Â Â Â Â Â Â */
> Â Â Â Â Â Â Â Âtrace_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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