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From:   Valentin Schneider <vschneid@redhat.com>
To:     John Stultz <jstultz@google.com>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     Arve =?utf-8?B?SGrDuG5uZXbDpWc=?= <arve@android.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        android-kernel-team <android-kernel-team@google.com>,
        John Stultz <jstultz@google.com>
Subject: Re: [RESEND][PATCH v2] sched/wait: Fix a kthread_park race with
 wait_woken()
In-Reply-To: <20230602212350.535358-1-jstultz@google.com>
References: <20230602212350.535358-1-jstultz@google.com>
Date:   Mon, 12 Jun 2023 16:28:07 +0100
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On 02/06/23 21:23, John Stultz wrote:
> From: Arve Hj=C3=B8nnev=C3=A5g <arve@android.com>
>
> kthread_park and wait_woken have a similar race that
> kthread_stop and wait_woken used to have before it was fixed in
> commit cb6538e740d7 ("sched/wait: Fix a kthread race with
> wait_woken()"). Extend that fix to also cover kthread_park.
>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: android-kernel-team <android-kernel-team@google.com>
> Signed-off-by: Arve Hj=C3=B8nnev=C3=A5g <arve@android.com>
> [jstultz: Made changes suggested by Peter to optimize
>  memory loads]
> Signed-off-by: John Stultz <jstultz@google.com>

Funny one, IIUC this will happen with any wakeup of a wait_woken()-user
that doesn't touch the wait_queue entry.

Reviewed-by: Valentin Schneider <vschneid@redhat.com>

I do have on question: what about signals? Browsing through wait_woken()
users, it looks like a few of them have a wait-loop break condition hinging
on signal_pending() (iio_buffer_read() is one of them). signal_wake_up()
doesn't touch the wait_queue entry, so is that a similar issue or am I
looking at a non-problem?