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

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Message-ID: <CAPKp9uaqeShkedY1aJpgAhe3NWeHdKOdNQv8W_OwKj+nFMjaDg@mail.gmail.com>
Date:   Fri, 16 Oct 2020 14:52:18 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Jerome Brunet <jbrunet@...libre.com>,
        Sudeep Holla <sudeep.holla@....com>
Cc:     Ionela Voinescu <ionela.voinescu@....com>,
        Jassi Brar <jassisinghbrar@...il.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, Oct 16, 2020 at 1:15 PM 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
>


There seem to be some races. It always hangs in the hrtimer_cancel.
Logging some extra messages makes it progress for a while and finally
get stuck in the loop. I wonder if there is a race between cancel
and handler execution.

-- 
Regards,
Sudeep