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