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

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Message-ID: <20211110104510.GB20566@blackbody.suse.cz>
Date:   Wed, 10 Nov 2021 11:45:10 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        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>
Subject: Re: printk deadlock due to double lock attempt on current CPU's
 runqueue

On Wed, Nov 10, 2021 at 10:00:35AM +0100, Vincent Guittot <vincent.guittot@...aro.org> 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")

AFAIK, the warning is typically triggered on v5.13 when CPU throttling is
employeed. The cfs_rq list is mis-ordered but it shouldn't be fatal.
But the throttling currently (since v5.13 too) also exposes a race on
task_group removal that can lead to corrupting the cfs_rq list (another
thread [1], I'll leave this thread for printk (non)-locking).

HTH,
Michal

[1] https://lore.kernel.org/r/20211011172236.11223-1-mkoutny@suse.com/