diff mbox series

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

Message ID 20220420085303.100654-4-javierm@redhat.com (mailing list archive)
State Superseded, archived
Headers show
Series Fix some race conditions that exists between fbmem and sysfb | expand

Commit Message

Javier Martinez Canillas April 20, 2022, 8:53 a.m. UTC 
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@ffwll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---

(no changes since v1)

 drivers/video/fbdev/core/fbmem.c | 22 +++++++++++++++-------
 include/linux/fb.h               |  1 -
 2 files changed, 15 insertions(+), 8 deletions(-)

Comments

Thomas Zimmermann April 25, 2022, 8:27 a.m. UTC | #1 
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.

Best regards
Thomas

> 
> 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@ffwll.ch>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
> 
> (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 84427470367b..0bb459258df3 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1553,6 +1553,7 @@ 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;
> @@ -1585,12 +1586,23 @@ 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;
>   		}
>   	}
>   }
> @@ -1897,13 +1909,9 @@ 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 f95da1af9ff6..b781bc721113 100644
> --- a/include/linux/fb.h
> +++ b/include/linux/fb.h
> @@ -502,7 +502,6 @@ 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) {
Javier Martinez Canillas April 25, 2022, 8:37 a.m. UTC | #2 
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.
diff mbox series

Patch

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 84427470367b..0bb459258df3 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1553,6 +1553,7 @@  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;
@@ -1585,12 +1586,23 @@  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;
 		}
 	}
 }
@@ -1897,13 +1909,9 @@  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 f95da1af9ff6..b781bc721113 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -502,7 +502,6 @@  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) {