linux-kernel - Re: printk deadlock due to double lock attempt on current CPU's runqueue

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Date:   Fri, 12 Nov 2021 08:50:46 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Sultan Alsawaf <sultan@...neltoast.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Anton Vorontsov <anton@...msg.org>,
        Ben Segall <bsegall@...gle.com>,
        Colin Cross <ccross@...roid.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        dri-devel@...ts.freedesktop.org, Ingo Molnar <mingo@...hat.com>,
        John Ogness <john.ogness@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Mel Gorman <mgorman@...e.de>, Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Tony Luck <tony.luck@...el.com>, mkoutny@...e.com
Subject: Re: printk deadlock due to double lock attempt on current CPU's runqueue

On Wed, 10 Nov 2021 at 20:50, Sultan Alsawaf <sultan@...neltoast.com> wrote:
>
> On Wed, Nov 10, 2021 at 10:00:35AM +0100, Vincent Guittot wrote:
> > Is it the same SCHED_WARN_ON(rq->tmp_alone_branch !=
> > &rq->leaf_cfs_rq_list); that generates the deadlock on v5.15 too ?
> >
> > one remaining tmp_alone_branch warning has been fixed in v5.15 with
> > 2630cde26711 ("sched/fair: Add ancestors of unthrottled undecayed cfs_rq")
>
> I should clarify that I didn't actually reproduce the issue on v5.15; I just saw
> that the call chain leading to the deadlock still existed in v5.15 after looking
> through the code.

Thanks for the clarification

>
> Failing the SCHED_WARN_ON(rq->tmp_alone_branch != &rq->leaf_cfs_rq_list); assert
> is extremely rare in my experience, and I don't have a reproducer. It has only
> happened once after months of heavy usage (with lots of reboots too, so not with
> crazy high uptime).
>
> Sultan