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From:   John Ogness <john.ogness@linutronix.de>
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To:     Sultan Alsawaf <sultan@kerneltoast.com>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Peter Zijlstra <peterz@infradead.org>,
        Anton Vorontsov <anton@enomsg.org>,
        Ben Segall <bsegall@google.com>,
        Colin Cross <ccross@android.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        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
In-Reply-To: <YYwlnEQO0raDwS86@sultan-box.localdomain>
References: <YYrU2PdmdNkulWSM@sultan-box.localdomain>
 <20211109213847.GY174703@worktop.programming.kicks-ass.net>
 <YYuS1uNhxWOEX1Ci@phenom.ffwll.local>
 <87fss4wcgm.fsf@jogness.linutronix.de>
 <YYwlnEQO0raDwS86@sultan-box.localdomain>
Date:   Thu, 11 Nov 2021 09:34:00 +0106
Message-ID: <87k0hfqepb.fsf@jogness.linutronix.de>
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On 2021-11-10, Sultan Alsawaf <sultan@kerneltoast.com> wrote:
> On Wed, Nov 10, 2021 at 11:13:37AM +0106, John Ogness wrote:
>> 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.
>
> Although I'm unaware of any ongoing kthread printer work, I'm curious
> to know how a kthread approach wouldn't employ a try_to_wake_up() from
> directly inside printk(), since the try_to_wake_up() hit from inside
> the fbcon code is what caused my deadlock.

The kthread approach triggers irq_work from printk(). The kthread
printer is then woken from the irq_work.

John Ogness