linux-kernel - Re: [PATCH] linux/wait: Fix __wait_event

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Date:   Tue, 12 Jul 2022 07:42:43 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Bruno Goncalves <bgoncalv@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH] linux/wait: Fix __wait_event_hrtimeout for RT/DL tasks

On 05/07/22 09:41, Valentin Schneider wrote:
> On 27/06/22 11:50, Juri Lelli wrote:
> > Changes to hrtimer mode (potentially made by __hrtimer_init_sleeper on
> > PREEMPT_RT) are not visible to hrtimer_start_range_ns, thus not
> > accounted for by hrtimer_start_expires call paths. In particular,
> > __wait_event_hrtimeout suffers from this problem as we have, for
> > example:
> >
> > fs/aio.c::read_events
> >   wait_event_interruptible_hrtimeout
> >     __wait_event_hrtimeout
> >       hrtimer_init_sleeper_on_stack <- this might "mode |= HRTIMER_MODE_HARD"
> >                                        on RT if task runs at RT/DL priority
> >         hrtimer_start_range_ns
> >           WARN_ON_ONCE(!(mode & HRTIMER_MODE_HARD) ^ !timer->is_hard)
> >           fires since the latter doesn't see the change of mode done by
> >           init_sleeper
> >
> > Fix it by making __wait_event_hrtimeout call hrtimer_sleeper_start_expires,
> > which is aware of the special RT/DL case, instead of hrtimer_start_range_ns.
> >
> > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Reported-by: Bruno Goncalves <bgoncalv@...hat.com>
> > Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
> 
> Makes sense, that's now aligned with what e.g.
> schedule_hrtimer_range_clock() does.
> 
> Reviewed-by: Valentin Schneider <vschneid@...hat.com>

Thanks!

Gentle ping to the others about this one.

Best,
Juri