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

From: Sudeep Holla <sudeep.holla@arm.com>
To: Ionela Voinescu <ionela.voinescu@arm.com>,
	Sudeep Holla <sudeep.holla@arm.com>
Cc: Jassi Brar <jassisinghbrar@gmail.com>,
	Jerome Brunet <jbrunet@baylibre.com>,
	Kevin Hilman <khilman@baylibre.com>,
	"open list:ARM/Amlogic Meson..."
	<linux-amlogic@lists.infradead.org>, Da Xue <da@libre.computer>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mailbox: cancel timer before starting it
Date: Fri, 16 Oct 2020 12:52:02 +0100	[thread overview]
Message-ID: <CAPKp9uZu0MggcMGyxK__UeGRONo3VHHWrqKwjOtZt-pguS=6JA@mail.gmail.com> (raw)
In-Reply-To: <20201016085217.GA12323@arm.com>

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