fbdev: Restart conflicting fb removal loop when unregistering devices

Drivers that want to remove registered conflicting framebuffers prior to
register their own framebuffer, call to remove_conflicting_framebuffers().

This function takes the registration_lock mutex, to prevent a race 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 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.

But this could be unsafe, since the fbdev core is making assumptions about
what drivers may or may not do in their .remove callbacks. Allowing to run
these callbacks with the registration_lock held can cause deadlocks, since
the fbdev core has no control over what drivers do in their removal path.

A better solution is to drop the lock before platform_device_unregister(),
so unregister_framebuffer() can take it when called from the fbdev driver.
The lock is acquired again after the device has been unregistered and at
this point the removal loop can be restarted.

Since the conflicting framebuffer device has already been 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>
Link: https://patchwork.freedesktop.org/patch/msgid/20220511113039.1252432-1-javierm@redhat.com

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);
 

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) {