linux-kernel - Re: [PATCH v5 3/7] fbdev: Restart conflicting fb removal loop when unregistering devices

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Message-ID: <2156c28c-9c9c-555e-ce19-933cdc8d5e74@suse.de>
Date:   Wed, 11 May 2022 13:47:03 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Helge Deller <deller@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v5 3/7] fbdev: Restart conflicting fb removal loop when
 unregistering devices

Hi Javier

Am 11.05.22 um 13:30 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.
> 
> Since the framebuffer devices will already be removed, the loop would just
> finish when no more conflicting framebuffers are found.
> 
> Suggested-by: Daniel Vetter <daniel.vetter@...ll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>

I'd like to shrink this patchset. This looks like it can be merged 
immediately?

Best regards
Thomas

> ---
> 
> (no changes since v1)
> 
>   drivers/video/fbdev/core/fbmem.c | 22 +++++++++++++++-------
>   include/linux/fb.h               |  1 -
>   2 files changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index b445a7a00def..2fda5917c212 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1555,6 +1555,7 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
>   {
>   	int i;
>   
> +restart_removal:
>   	/* check all firmware fbs and kick off if the base addr overlaps */
>   	for_each_registered_fb(i) {
>   		struct apertures_struct *gen_aper;
> @@ -1587,12 +1588,23 @@ static void do_remove_conflicting_framebuffers(struct apertures_struct *a,
>   				pr_warn("fb%d: no device set\n", i);
>   				do_unregister_framebuffer(registered_fb[i]);
>   			} else if (dev_is_platform(device)) {
> -				registered_fb[i]->forced_out = true;
> +				/*
> +				 * Drop the lock because if the device is unregistered, its
> +				 * driver will call to unregister_framebuffer(), that takes
> +				 * this lock.
> +				 */
> +				mutex_unlock(&registration_lock);
>   				platform_device_unregister(to_platform_device(device));
> +				mutex_lock(&registration_lock);
>   			} else {
>   				pr_warn("fb%d: cannot remove device\n", i);
>   				do_unregister_framebuffer(registered_fb[i]);
>   			}
> +			/*
> +			 * Restart the removal loop now that the device has been
> +			 * unregistered and its associated framebuffer gone.
> +			 */
> +			goto restart_removal;
>   		}
>   	}
>   }
> @@ -1899,13 +1911,9 @@ EXPORT_SYMBOL(register_framebuffer);
>   void
>   unregister_framebuffer(struct fb_info *fb_info)
>   {
> -	bool forced_out = fb_info->forced_out;
> -
> -	if (!forced_out)
> -		mutex_lock(&registration_lock);
> +	mutex_lock(&registration_lock);
>   	do_unregister_framebuffer(fb_info);
> -	if (!forced_out)
> -		mutex_unlock(&registration_lock);
> +	mutex_unlock(&registration_lock);
>   }
>   EXPORT_SYMBOL(unregister_framebuffer);
>   
> diff --git a/include/linux/fb.h b/include/linux/fb.h
> index 69c67c70fa78..bbe1e4571899 100644
> --- a/include/linux/fb.h
> +++ b/include/linux/fb.h
> @@ -511,7 +511,6 @@ struct fb_info {
>   	} *apertures;
>   
>   	bool skip_vt_switch; /* no VT switch on suspend/resume required */
> -	bool forced_out; /* set when being removed by another driver */
>   };
>   
>   static inline struct apertures_struct *alloc_apertures(unsigned int max_num) {

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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