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

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Message-ID: <YYvHW1OpN1L2uInb@phenom.ffwll.local>
Date:   Wed, 10 Nov 2021 14:21:31 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        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>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: printk deadlock due to double lock attempt on current CPU's
 runqueue

On Wed, Nov 10, 2021 at 12:20:05PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 10, 2021 at 11:50:38AM +0100, Petr Mladek wrote:
> > On Tue 2021-11-09 12:06:48, Sultan Alsawaf wrote:
> > > Hi,
> > > 
> > > I encountered a printk deadlock on 5.13 which appears to still affect the latest
> > > kernel. The deadlock occurs due to printk being used while having the current
> > > CPU's runqueue locked, and the underlying framebuffer console attempting to lock
> > > the same runqueue when printk tries to flush the log buffer.
> > > 
> > > I'm not sure what the *correct* solution is here (don't use printk while having
> > > a runqueue locked? don't use schedule_work() from the fbcon path? tell printk
> > > to use one of its lock-less backends?), so I've cc'd all the relevant folks.
> > 
> > At the moment, printk_deferred() could be used here. It defers the
> > console handling via irq_work().
> 
> I think I've rejected that patch at least twice now :-) John's printk
> stuff will really land real soon now, right.

Yeah whacking all affected prinkt callers just because of fbcon does not
sound like a good idea to me either.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch