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

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Message-ID: <xhsmhtu7weytl.mognet@vschneid.remote.csb>
Date:   Tue, 05 Jul 2022 09:41:26 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Juri Lelli <juri.lelli@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        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 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>