Message-ID: <20110905103925.GC5466@quack.suse.cz>
Date:	Mon, 5 Sep 2011 12:39:25 +0200
From:	Jan Kara <jack@...e.cz>
To:	"kautuk.c @samsung.com" <consul.kautuk@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jaxboe@...ionio.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Dave Chinner <dchinner@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/backing-dev.c: Call del_timer_sync instead of
 del_timer

On Mon 05-09-11 11:19:46, kautuk.c @samsung.com wrote:
> On Fri, Sep 2, 2011 at 8:44 PM, Jan Kara <jack@...e.cz> wrote:
> > 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...
> >
> On second thought:
> In the case that the timer_fn of the default bdi causes the
> bdi_forker_thread to wake up, why waste CPU time on one more loop when we
> could know convincingly that we would want to sleep ?
  OK, I don't care much whether we have there del_timer() or
del_timer_sync(). Let me just say that the race you are afraid of is
probably not going to happen in practice so I'm not sure it's valid to be
afraid of CPU cycles being burned needlessly. The timer is armed when an
dirty inode is first attached to default bdi's dirty list. Then the default
bdi flusher thread would have to be woken up so that following happens:
	CPU1				CPU2
  timer fires -> wakeup_timer_fn()
					bdi_forker_thread()
					  del_timer(&me->wakeup_timer);
					  wb_do_writeback(me, 0);
					  ...
					  set_current_state(TASK_INTERRUPTIBLE);
  wake_up_process(default_backing_dev_info.wb.task);

  Especially wb_do_writeback() is going to take a long time so just that
single thing makes the race unlikely. Given del_timer_sync() is slightly
more costly than del_timer() even for unarmed timer, it is questionable
whether (chance race happens * CPU spent in extra loop) > (extra CPU spent
in del_timer_sync() * frequency that code is executed in
bdi_forker_thread())...

								Honza

> Of course, if any of the other BDIs are scheduling work to the default
> the default thread
> will wake up reliably as you mentioned.
> But, in the case that the race between the default BDI's own timer_fn
> (me->wakeup_timer)
> on one CPU with the code in bdi_forker_thread on another CPU happens,
> we will end up in
> one more loop which will result in more CPU usage when we could
> actually just go to sleep in
> the current iteration of the loop if no work is found on its own bdi
> list (i.e., me->bdi->work_list).
> 
> 
> >> On Fri, Sep 2, 2011 at 5:14 PM, kautuk.c @samsung.com
> >> <consul.kautuk@...il.com> wrote:
> >> > Hi,
> >> >
> >> > On Fri, Sep 2, 2011 at 4:51 PM, Jan Kara <jack@...e.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@...ux-foundation.org> wrote:
> >> >>> > On Thu,  1 Sep 2011 21:27:02 +0530
> >> >>> > Kautuk Consul <consul.kautuk@...il.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@...e.cz>
> >> >> SUSE Labs, CR
> >> >>
> >> >
> > --
> > Jan Kara <jack@...e.cz>
> > SUSE Labs, CR
> >
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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