On Fri 16 Oct 2020 at 19:33, Jassi Brar <jassisinghb...@gmail.com> wrote:
> On Fri, Oct 16, 2020 at 4:00 AM Jerome Brunet <jbru...@baylibre.com> wrote: >> >> >> On Fri 16 Oct 2020 at 10:52, Ionela Voinescu <ionela.voine...@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. >> > >> >> 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 >> > If we are running in the callback path, hrtimer_try_to_cancel will > return -1, in which case we could skip hrtimer_start. > Anyways, I think simply checking for hrtimer_active should effect the same. > I have submitted a patch, of course not tested. Yes it sloves this race but ... If a race is possible between a timer callback rescheduling itself (which is not that uncommon) and another thread trying to cancel it, maybe there is something worth fixing in hrtimer ? Also, mailbox calls hrtimer_cancel() in unregister ... are we confident this would work ? Any fix is by me - yours avoid killing and restarting the timer :) but it feels like we are working around an issue that might bite us back later on. > > Thanks