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 03:15:32PM +0900, Namjae Jeon wrote:
>> 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 ?
>
> wakeup_timer_fn() holds the wb_lock across the check for bdi->dev !=
> NULL and the call to trace_writeback_wake_forker_thread(), so no.

The following case is not possible with racing of __mark_inode_dirty  ?
--------------------------------------------------------------------------------------------------
spin_lock
bdi->dev = NULL;
spin_unlock
                              spin_lock
                               ......
                               ......
                               trace_writeback_wake_forker_thread(bdi);
                               spin_unlock
-----------------------------------------------------------------------------
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