linux-kernel - Re: [PATCH] mailbox: cancel timer before starting it

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Message-ID: <1jft6ej91c.fsf@starbuckisacylon.baylibre.com>
Date:   Fri, 16 Oct 2020 20:54:55 +0200
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Jassi Brar <jassisinghbrar@...il.com>
Cc:     Ionela Voinescu <ionela.voinescu@....com>,
        Kevin Hilman <khilman@...libre.com>,
        "open list\:ARM\/Amlogic Meson..." 
        <linux-amlogic@...ts.infradead.org>, Da Xue <da@...re.computer>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mailbox: cancel timer before starting it


On Fri 16 Oct 2020 at 19:33, Jassi Brar <jassisinghbrar@...il.com> wrote:

> On Fri, Oct 16, 2020 at 4:00 AM Jerome Brunet <jbrunet@...libre.com> wrote:
>>
>>
>> On Fri 16 Oct 2020 at 10:52, Ionela Voinescu <ionela.voinescu@....com> wrote:
>>
>> > On Thursday 15 Oct 2020 at 13:45:54 (-0500), Jassi Brar wrote:
>> > [..]
>> >> > >> --- a/drivers/mailbox/mailbox.c
>> >> > >> +++ b/drivers/mailbox/mailbox.c
>> >> > >> @@ -82,9 +82,13 @@ static void msg_submit(struct mbox_chan *chan)
>> >> > >>  exit:
>> >> > >>      spin_unlock_irqrestore(&chan->lock, flags);
>> >> > >>
>> >> > >> -    if (!err && (chan->txdone_method & TXDONE_BY_POLL))
>> >> > >> -            /* kick start the timer immediately to avoid delays */
>> >> > >> +    if (!err && (chan->txdone_method & TXDONE_BY_POLL)) {
>> >> > >> +            /* Disable the timer if already active ... */
>> >> > >> +            hrtimer_cancel(&chan->mbox->poll_hrt);
>> >> > >> +
>> >> > >> +            /* ... and kick start it immediately to avoid delays */
>> >> > >>              hrtimer_start(&chan->mbox->poll_hrt, 0, HRTIMER_MODE_REL);
>> >> > >> +    }
>> >> > >>  }
>> >> > >>
>> >> > >>  static void tx_tick(struct mbox_chan *chan, int r)
>> >> > >
>> >> > > I've tracked a regression back to this commit. Details to reproduce:
>> >> >
>> >> > Hi Ionela,
>> >> >
>> >> > I don't have access to your platform and I don't get what is going on
>> >> > from the log below.
>> >> >
>> >> > Could you please give us a bit more details about what is going on ?
>> >> >
>> >> > All this patch does is add hrtimer_cancel().
>> >> > * It is needed if the timer had already been started, which is
>> >> >   appropriate AFAIU
>> >> > * It is a NO-OP is the timer is not active.
>> >> >
>> >> Can you please try using hrtimer_try_to_cancel() instead ?
>> >>
>> >
>> > Yes, using hrtimer_try_to_cancel() instead works for me. But doesn't
>> > this limit how effective this change is? AFAIU, this will possibly only
>> > reduce the chances for the race condition, but not solve it.
>> >
>>
>> It is also my understanding, hrtimer_try_to_cancel() would remove a
>> timer which as not already started but would return withtout doing
>> anything if the callback is already running ... which is the original
>> problem
>>
> If we are running in the callback path, hrtimer_try_to_cancel will
> return -1, in which case we could skip hrtimer_start.
> Anyways, I think simply checking for hrtimer_active should effect the same.
> I have submitted a patch, of course not tested.

Yes it sloves this race but ...

If a race is possible between a timer callback rescheduling itself (which
is not that uncommon) and another thread trying to cancel it, maybe
there is something worth fixing in hrtimer ? Also, mailbox calls
hrtimer_cancel() in unregister ... are we confident this would work ?

Any fix is by me - yours avoid killing and restarting the timer :) but
it feels like we are working around an issue that might bite us back
later on.

>
> Thanks