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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87fss4wcgm.fsf@jogness.linutronix.de>
Date:   Wed, 10 Nov 2021 11:13:37 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Daniel Vetter <daniel@...ll.ch>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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>,
        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>,
        Vincent Guittot <vincent.guittot@...aro.org>, mkoutny@...e.com
Subject: Re: printk deadlock due to double lock attempt on current CPU's
 runqueue

On 2021-11-10, Daniel Vetter <daniel@...ll.ch> wrote:
> I'm a bit out of the loop but from lwn articles my understanding is
> that part of upstreaming from -rt we no longer have the explicit "I'm
> a safe console for direct printing" opt-in. Which I get from a
> backwards compat pov, but I still think for at least fbcon we really
> should never attempt a direct printk con->write, it's just all around
> terrible.

Right now we don't have an explicit "I'm a safe console for direct
printing" option. Right now all printing is direct. But it sounds to me
that we should add this console flag when we introduce kthread printers.

> So yeah for fbcon at least I think we really should throw out direct
> con->write from printk completely.

Even after we introduce kthread printers, there will still be situations
where direct printing is used: booting (before kthreads exist) and
shutdown/suspend/crash situations, when the kthreads may not be
active.

I will introduce a console flag so that consoles can opt-out for direct
printing. (opt-out rather than opt-in is probably easier, since there
are only a few that would need to opt-out).

Since kthread printers do not yet exist (hoping to get them in for
5.17), I am not sure how we should address the reported bug for existing
kernels.

John Ogness