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

Sudeep Holla
2020-10-16T11:52:18Z

On Fri, Oct 16, 2020 at 12:32 PM 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.
>

Indeed, I tried the same and got a lot of -ETIME failures. hrtimer_cancel
uses hrtimer_try_to_cancel in loop until it succeeds so that the following
hrtimer_start can be done w/o any pending timers.

--
Regards,
Sudeep