linux-kernel - Re: [PATCH v3 2/2] sched/tracing: Report TASK_RTLOCK_WAIT tasks as TASK

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Date:   Sun, 10 Apr 2022 00:42:24 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     stable@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Gladkov <legion@...nel.org>,
        "Kenta.Tada@...y.com" <Kenta.Tada@...y.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ed Tsai <ed.tsai@...iatek.com>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v3 2/2] sched/tracing: Report TASK_RTLOCK_WAIT tasks as
 TASK_UNINTERRUPTIBLE

+CC stable

On 01/20/22 16:25, Valentin Schneider wrote:
> TASK_RTLOCK_WAIT currently isn't part of TASK_REPORT, thus a task blocking
> on an rtlock will appear as having a task state == 0, IOW TASK_RUNNING.
> 
> The actual state is saved in p->saved_state, but reading it after reading
> p->__state has a few issues:
> o that could still be TASK_RUNNING in the case of e.g. rt_spin_lock
> o ttwu_state_match() might have changed that to TASK_RUNNING
> 
> As pointed out by Eric, adding TASK_RTLOCK_WAIT to TASK_REPORT implies
> exposing a new state to userspace tools which way not know what to do with
> them. The only information that needs to be conveyed here is that a task is
> waiting on an rt_mutex, which matches TASK_UNINTERRUPTIBLE - there's no
> need for a new state.
> 
> Reported-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>

Any objection for this to be picked up by stable? We care about Patch 1 only in
this series for stable, but it seems sensible to pick this one too, no strong
feeling if it is omitted though.

AFAICT it seems the problem dates back since commit:

	1593baab910d ("sched/debug: Implement consistent task-state printing")

or even before. I think v4.14+ is good enough.

Thoughts?

Thanks!

--
Qais Yousef