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

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Date:   Fri, 16 Oct 2020 11:00:37 +0200
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Ionela Voinescu <ionela.voinescu@....com>,
        Jassi Brar <jassisinghbrar@...il.com>
Cc:     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 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


> Thanks,
> Ionela.
>
>> thanks