Re: [PATCH v3 3/5] fbdev: Restart conflicting fb removal loop when unregistering devices

[Javier Martinez Canillas <javierm@redhat.com>]

Hello Thomas,

Thanks for the feedback.

On 4/25/22 10:27, Thomas Zimmermann wrote:
> Hi
> 
> Am 20.04.22 um 10:53 schrieb Javier Martinez Canillas:
>> Drivers that want to remove registered conflicting framebuffers prior to
>> register their own framebuffer, calls remove_conflicting_framebuffers().
>>
>> This function takes the registration_lock mutex, to prevent a races when
>> drivers register framebuffer devices. But if a conflicting framebuffer
>> device is found, the underlaying platform device is unregistered and this
>> will lead to the platform driver .remove callback to be called, which in
>> turn will call to the unregister_framebuffer() that takes the same lock.
>>
>> To prevent this, a struct fb_info.forced_out field was used as indication
>> to unregister_framebuffer() whether the mutex has to be grabbed or not.
>>
>> A cleaner solution is to drop the lock before platform_device_unregister()
>> so unregister_framebuffer() can take it when called from the fbdev driver,
>> and just grab the lock again after the device has been registered and do
>> a removal loop restart.
> 
> I don't see how this patch improves the situation. So far, 
> do_remove_conflicting_framebuffers() had no business in maintaining 
> locks. And now it's doing this in in a goto-loop where it keeps 
> getting/dropping locks.  That's asking for bugs IMHO.
>

It's true that do_remove_conflicting_framebuffers() gets more complicated
with all the locks release/re-acquire but OTOH unregister_framebuffer()
doesn't do conditionally locking, and more importantly the drivers .remove
callback isn't called with the lock held, which IMHO is also quite fragile.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat