lkml.org 
[lkml]   [2011]   [Sep]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 1/1] mm/backing-dev.c: Call del_timer_sync instead of del_timer
    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
    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2011-09-02 17:17    [W:0.029 / U:34.584 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site