[PATCH 3/9] fsfreeze: Prevent emergency thaw from looping infinitely

["Fernando Luis Vázquez Cao" <fernando_b1@lab.ntt.co.jp>]

The thawing of a filesystem through sysrq-j loops infinitely as it
incorrectly detects a thawed filesytsem as frozen and tries to
unfreeze repeatedly. This is a regression caused by
4504230a71566785a05d3e6b53fa1ee071b864eb ("freeze_bdev: grab active
reference to frozen superblocks") in that it no longer returned
-EINVAL for superblocks that were not frozen.

Return -EINVAL when the filesystem is already unfrozen to avoid this
problem.

Cc: Josef Bacik <jbacik@fusionio.com>
Cc: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
---

diff -urNp vfs-orig/fs/block_dev.c vfs/fs/block_dev.c
--- vfs-orig/fs/block_dev.c	2012-07-04 18:57:54.000000000 +0900
+++ vfs/fs/block_dev.c	2012-07-12 13:22:38.124815295 +0900
@@ -319,22 +319,17 @@ int thaw_bdev(struct block_device *bdev,
 	if (!bdev->bd_fsfreeze_count)
 		goto out;
 
-	error = 0;
-	if (--bdev->bd_fsfreeze_count > 0)
-		goto out;
-
-	if (!sb)
+	if (--bdev->bd_fsfreeze_count > 0 || !sb) {
+		error = 0;
 		goto out;
+	}
 
 	error = thaw_super(sb);
-	if (error) {
+	if (error)
 		bdev->bd_fsfreeze_count++;
-		mutex_unlock(&bdev->bd_fsfreeze_mutex);
-		return error;
-	}
 out:
 	mutex_unlock(&bdev->bd_fsfreeze_mutex);
-	return 0;
+	return error;
 }
 EXPORT_SYMBOL(thaw_bdev);