linux-kernel - [RFC PATCH v4 06/11] fbdev: Restart conflicting fb removal loop when unregistering devices

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Message-Id: <20220429084253.1085911-7-javierm@redhat.com>
Date:   Fri, 29 Apr 2022 10:42:48 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Javier Martinez Canillas <javierm@...hat.com>,
        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: [RFC PATCH v4 06/11] fbdev: Restart conflicting fb removal loop when unregistering devices

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>
---

(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) {
-- 
2.35.1