Re: [PATCH v2] fbdev: Fix incorrect page mapping clearance at fb_deferred_io_release()

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Added to drm-misc-fixes. Thanks a lot for the patch.

Am 08.03.23 um 11:50 schrieb Takashi Iwai:
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@xxxxxxxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
v1->v2:
* Rename to fb_deferred_io_lastclose()
* Rename the new field from opens to open_count
* Removed unused variable
* More comments about fb_info locking

  drivers/video/fbdev/core/fb_defio.c | 17 +++++++++++++----
  include/linux/fb.h                  |  1 +
  2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
index aa5f059d0222..274f5d0fa247 100644
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/drivers/video/fbdev/core/fb_defio.c
@@ -305,17 +305,18 @@ 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 */
@@ -324,13 +325,21 @@ 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);
diff --git a/include/linux/fb.h b/include/linux/fb.h
index d8d20514ea05..02d09cb57f6c 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 */

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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