Re: [PATCH] mailbox: cancel timer before starting it

Jerome Brunet
2020-10-16T09:00:44Z

On Fri 16 Oct 2020 at 10:52, Ionela Voinescu <ionela.voinescu@arm.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

> Thanks,
> Ionela.
>
>> thanks