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

[Jan Kara <jack@suse.cz>]

On Fri 02-09-11 17:32:35, kautuk.c @samsung.com wrote:
> Hi Jan,
> 
> I looked at that other patch you just sent.
> 
> I think that the task state problem can still happen in that case as the setting
> of the task state is not protected by any lock and the timer callback can be
> executing on another CPU at that time.
> 
> Am I right about this ?
  Yes, the cleanup is not meant to change the scenario you describe - as I
said, there's no point in protecting against it as it's harmless...

								Honza

> On Fri, Sep 2, 2011 at 5:14 PM, kautuk.c @samsung.com
> <consul.kautuk@gmail.com> wrote:
> > 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
> >>
> >
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR