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

From: John Ogness <john.ogness@linutronix.de>
To: Daniel Vetter <daniel@ffwll.ch>, Peter Zijlstra <peterz@infradead.org>
Cc: Sultan Alsawaf <sultan@kerneltoast.com>,
	Anton Vorontsov <anton@enomsg.org>,
	Ben Segall <bsegall@google.com>, Colin Cross <ccross@android.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@linux.ie>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	dri-devel@lists.freedesktop.org, Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Kees Cook <keescook@chromium.org>,
	linux-kernel@vger.kernel.org,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>, Mel Gorman <mgorman@suse.de>,
	Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Tony Luck <tony.luck@intel.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	mkoutny@suse.com
Subject: Re: printk deadlock due to double lock attempt on current CPU's runqueue
Date: Wed, 10 Nov 2021 11:13:37 +0106	[thread overview]
Message-ID: <87fss4wcgm.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <YYuS1uNhxWOEX1Ci@phenom.ffwll.local>

On 2021-11-10, Daniel Vetter <daniel@ffwll.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