linux-kernel - Re: [PATCH] drm: fb-helper: Avoid nesting spinlock_t into raw_spinlock

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Message-ID: <87o8382j5s.fsf@jogness.linutronix.de>
Date:   Tue, 15 Feb 2022 17:29:27 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Sebastian Siewior <bigeasy@...utronix.de>,
        Jiri Kosina <jikos@...nel.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: fb-helper: Avoid nesting spinlock_t into
 raw_spinlock_t

On 2022-02-15, Sebastian Siewior <bigeasy@...utronix.de> wrote:
>> From: Jiri Kosina <jkosina@...e.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