Re: [PATCH 1/1] mm/backing-dev.c: Call del_timer_sync instead of del_timer

["kautuk.c @samsung.com" <consul.kautuk@gmail.com>]

Hi,

On Fri, Sep 2, 2011 at 4:51 PM, Jan Kara <jack@suse.cz> wrote:
>  Hello,
>
> On Fri 02-09-11 10:47:03, kautuk.c @samsung.com wrote:
>> On Fri, Sep 2, 2011 at 3:03 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
>> > On Thu,  1 Sep 2011 21:27:02 +0530
>> > Kautuk Consul <consul.kautuk@gmail.com> wrote:
>> >
>> >> This is important for SMP scenario, to check whether the timer
>> >> callback is executing on another CPU when we are deleting the
>> >> timer.
>> >>
>> >
>> > I don't see why?
>> >
>> >> index d6edf8d..754b35a 100644
>> >> --- a/mm/backing-dev.c
>> >> +++ b/mm/backing-dev.c
>> >> @@ -385,7 +385,7 @@ static int bdi_forker_thread(void *ptr)
>> >>                * dirty data on the default backing_dev_info
>> >>                */
>> >>               if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) {
>> >> -                     del_timer(&me->wakeup_timer);
>> >> +                     del_timer_sync(&me->wakeup_timer);
>> >>                       wb_do_writeback(me, 0);
>> >>               }
>> >
>> > It isn't a use-after-free fix: bdi_unregister() safely shoots down any
>> > running timer.
>> >
>>
>> In the situation that we do a del_timer at the same time that the
>> wakeup_timer_fn is
>> executing on another CPU, there is one tiny possible problem:
>> 1)  The wakeup_timer_fn will call wake_up_process on the bdi-default thread.
>>       This will set the bdi-default thread's state to TASK_RUNNING.
>> 2)  However, the code in bdi_writeback_thread() sets the state of the
>> bdi-default process
>>     to TASK_INTERRUPTIBLE as it intends to sleep later.
>>
>> If 2) happens before 1), then the bdi_forker_thread will not sleep
>> inside schedule as is the intention of the bdi_forker_thread() code.
>  OK, I agree the code in bdi_forker_thread() might use some straightening
> up wrt. task state handling but is what you decribe really an issue? Sure
> the task won't go to sleep but the whole effect is that it will just loop
> once more to find out there's nothing to do and then go to sleep - not a
> bug deal... Or am I missing something?

Yes, you are right.
I was studying the code and I found this inconsistency.
Anyways, if there is NO_ACTION it will just loop and go to sleep again.
I just posted this because I felt that the code was not achieving the logic
that was intended in terms of sleeps and wakeups.

I am currently trying to study the other patches you have just sent.

>
>> This protection is not achieved even by acquiring spinlocks before
>> setting the task->state
>> as the spinlock used in wakeup_timer_fn is &bdi->wb_lock whereas the code in
>> bdi_forker_thread acquires &bdi_lock which is a different spin_lock.
>>
>> Am I correct in concluding this ?
>
>                                                                Honza
> --
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR
>

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