freeze_bdev() with the device which is mounted as read only
does not change sb->s_frozen from SB_UNFROZEN to SB_FREEZE_TRANS.

Because of this behavior, xfs_freeze can break read-only XFS filesystem.

Because xfs_thaw does nothing for the filesystem whose sb->s_frozen is
SB_UNFROZEN. So freezed readonly XFS filesystem will never be unfreezed.
Then we cannot do any unmount/remount operations for that filesystem.

This patch updates sb->s_frozen when freeze_bdev() is called for read-only
mounted device, too.

Cc:  Tim Shimmin <xfs-masters@oss.sgi.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>

---
 fs/buffer.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

Index: 2.6-git/fs/buffer.c
===================================================================
--- 2.6-git.orig/fs/buffer.c
+++ 2.6-git/fs/buffer.c
@@ -190,19 +190,22 @@ struct super_block *freeze_bdev(struct b
 
 	down(&bdev->bd_mount_sem);
 	sb = get_super(bdev);
-	if (sb && !(sb->s_flags & MS_RDONLY)) {
+	if (sb) {
 		sb->s_frozen = SB_FREEZE_WRITE;
 		smp_wmb();
 
-		__fsync_super(sb);
+		if (!(sb->s_flags & MS_RDONLY))
+			__fsync_super(sb);
 
 		sb->s_frozen = SB_FREEZE_TRANS;
 		smp_wmb();
 
-		sync_blockdev(sb->s_bdev);
+		if (!(sb->s_flags & MS_RDONLY)) {
+			sync_blockdev(sb->s_bdev);
 
-		if (sb->s_op->write_super_lockfs)
-			sb->s_op->write_super_lockfs(sb);
+			if (sb->s_op->write_super_lockfs)
+				sb->s_op->write_super_lockfs(sb);
+		}
 	}
 
 	sync_blockdev(bdev);
-
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