Re: [PATCH] drm: fb-helper: Avoid nesting spinlock_t into raw_spinlock_t

[John Ogness]

On 2022-02-15, Sebastian Siewior <bigeasy@xxxxxxxxxxxxx> wrote:
>> From: Jiri Kosina <jkosina@xxxxxxx>
>> 
>> 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@xxxxxxxxxxxxx