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

Jassi Brar
2020-10-15T18:46:09Z

On Thu, Oct 15, 2020 at 8:58 AM Jerome Brunet <jbrunet@baylibre.com> wrote:
>
>
> On Thu 15 Oct 2020 at 15:46, Ionela Voinescu <ionela.voinescu@arm.com> wrote:
>
> > Hi guys,
> >
> > On Wednesday 23 Sep 2020 at 14:39:16 (+0200), Jerome Brunet wrote:
> >> If the txdone is done by polling, it is possible for msg_submit() to start
> >> the timer while txdone_hrtimer() callback is running. If the timer needs
> >> recheduling, it could already be enqueued by the time hrtimer_forward_now()
> >> is called, leading hrtimer to loudly complain.
> >>
> >> WARNING: CPU: 3 PID: 74 at kernel/time/hrtimer.c:932 hrtimer_forward+0xc4/0x110
> >> CPU: 3 PID: 74 Comm: kworker/u8:1 Not tainted 5.9.0-rc2-00236-gd3520067d01c-dirty #5
> >> Hardware name: Libre Computer AML-S805X-AC (DT)
> >> Workqueue: events_freezable_power_ thermal_zone_device_check
> >> pstate: 20000085 (nzCv daIf -PAN -UAO BTYPE=--)
> >> pc : hrtimer_forward+0xc4/0x110
> >> lr : txdone_hrtimer+0xf8/0x118
> >> [...]
> >>
> >> Canceling the timer before starting it ensure that the timer callback is
> >> not running when the timer is started, solving this race condition.
> >>
> >> Fixes: 0cc67945ea59 ("mailbox: switch to hrtimer for tx_complete polling")
> >> Reported-by: Da Xue <da@libre.computer>
> >> Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
> >> ---
> >> drivers/mailbox/mailbox.c | 8 ++++++--
> >> 1 file changed, 6 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
> >> index 0b821a5b2db8..34f9ab01caef 100644
> >> --- 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 ?

thanks