fbdev: Fix incorrect page mapping clearance at fb_deferred_io_release()

The recent fix for the deferred I/O by the commit
  3efc61d ("fbdev: Fix invalid page access after closing deferred I/O devices")
caused a regression when the same fb device is opened/closed while
it's being used.  It resulted in a frozen screen even if something
is redrawn there after the close.  The breakage is because the patch
was made under a wrong assumption of a single open; in the current
code, fb_deferred_io_release() cleans up the page mapping of the
pageref list and it calls cancel_delayed_work_sync() unconditionally,
where both are no correct behavior for multiple opens.

This patch adds a refcount for the opens of the device, and applies
the cleanup only when all files get closed.

As both fb_deferred_io_open() and _close() are called always in the
fb_info lock (mutex), it's safe to use the normal int for the
refcounting.

Also, a useless BUG_ON() is dropped.

Fixes: 3efc61d ("fbdev: Fix invalid page access after closing deferred I/O devices")
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230308105012.1845-1-tiwai@suse.de

Author: tiwai

drivers/video/fbdev/core/fb_defio.c

@@ -309,17 +309,18 @@ void fb_deferred_io_open(struct fb_info *info,
 			 struct inode *inode,
 			 struct file *file)
 {
+	struct fb_deferred_io *fbdefio = info->fbdefio;
+
 	file->f_mapping->a_ops = &fb_deferred_io_aops;
+	fbdefio->open_count++;
 }
 EXPORT_SYMBOL_GPL(fb_deferred_io_open);
 
-void fb_deferred_io_release(struct fb_info *info)
+static void fb_deferred_io_lastclose(struct fb_info *info)
 {
-	struct fb_deferred_io *fbdefio = info->fbdefio;
 	struct page *page;
 	int i;
 
-	BUG_ON(!fbdefio);
 	cancel_delayed_work_sync(&info->deferred_work);
 
 	/* clear out the mapping that we setup */
@@ -328,13 +329,21 @@ void fb_deferred_io_release(struct fb_info *info)
 		page->mapping = NULL;
 	}
 }
+
+void fb_deferred_io_release(struct fb_info *info)
+{
+	struct fb_deferred_io *fbdefio = info->fbdefio;
+
+	if (!--fbdefio->open_count)
+		fb_deferred_io_lastclose(info);
+}
 EXPORT_SYMBOL_GPL(fb_deferred_io_release);
 
 void fb_deferred_io_cleanup(struct fb_info *info)
 {
 	struct fb_deferred_io *fbdefio = info->fbdefio;
 
-	fb_deferred_io_release(info);
+	fb_deferred_io_lastclose(info);
 
 	kvfree(info->pagerefs);
 	mutex_destroy(&fbdefio->lock);

include/linux/fb.h

@@ -212,6 +212,7 @@ struct fb_deferred_io {
 	/* delay between mkwrite and deferred handler */
 	unsigned long delay;
 	bool sort_pagereflist; /* sort pagelist by offset */
+	int open_count; /* number of opened files; protected by fb_info lock */
 	struct mutex lock; /* mutex that protects the pageref list */
 	struct list_head pagereflist; /* list of pagerefs for touched pages */
 	/* callback */