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From:   John Ogness <john.ogness@linutronix.de>
To:     Sebastian Siewior <bigeasy@linutronix.de>,
        Jiri Kosina <jikos@kernel.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] drm: fb-helper: Avoid nesting spinlock_t into
 raw_spinlock_t
In-Reply-To: <YgvLdvPihuQ9KZ6/@linutronix.de>
References: <nycvar.YFH.7.76.2202151640200.11721@cbobk.fhfr.pm>
 <YgvLdvPihuQ9KZ6/@linutronix.de>
Date:   Tue, 15 Feb 2022 17:29:27 +0106
Message-ID: <87o8382j5s.fsf@jogness.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 2022-02-15, Sebastian Siewior <bigeasy@linutronix.de> wrote:
>> From: Jiri Kosina <jkosina@suse.cz>
>> 
>> drm_fb_helper_damage() is acquiring spinlock_t (helper->damage_lock), 
>> while it can be called from contexts where raw_spinlock_t is held (e.g. 
>> console_owner lock obtained on vprintk_emit() codepath).
>> 
>> As the critical sections protected by damage_lock are super-tiny, let's 
>> fix this by converting it to raw_spinlock_t in order not to violate 
>> PREEMPT_RT-imposed lock nesting rules.
>> 
>> This fixes the splat below.
>> 
>>  =============================
>>  [ BUG: Invalid wait context ]
>>  5.17.0-rc4-00002-gd567f5db412e #1 Not tainted
>
> rc4. Is this also the case in the RT tree which includes John's printk
> changes?

In the RT tree, the fbcon's write() callback is only called in
preemptible() contexts. So this is only a mainline issue.

The series I recently posted to LKML [0] should also address this issue
(if/when it gets accepted).

John

[0] https://lore.kernel.org/lkml/20220207194323.273637-1-john.ogness@linutronix.de