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

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Message-ID: <41362e99-820c-727c-019a-11ec5f38083c@redhat.com>
Date:   Mon, 25 Apr 2022 10:37:43 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        Changcheng Deng <deng.changcheng@....com.cn>,
        Daniel Vetter <daniel@...ll.ch>, Helge Deller <deller@....de>,
        Sam Ravnborg <sam@...nborg.org>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        linux-fbdev@...r.kernel.org
Subject: Re: [PATCH v3 3/5] fbdev: Restart conflicting fb removal loop when
 unregistering devices

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