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

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