fbdev: Fix incorrect page mapping clearance at fb_deferred_io_release()

The recent fix for the deferred I/O by the commit
  3efc61d95259 ("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: 3efc61d95259 ("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

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/fb.h b/include/linux/fb.h
index 73eb1f85ea8e..05e40fcc7696 100644
--- a/include/linux/fb.h
+++ b/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 */