update sb->s_frozen when freezing read-only mounted device, too

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.

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

Index: 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);

I think this fix is valid, but it might be a tad cleaner to just set s_frozen to SB_FREEZE_TRANS directly in a separate branch, ala:

struct super_block *freeze_bdev(struct block_device *bdev)

{
        struct super_block *sb;

        down(&bdev->bd_mount_sem);
        sb = get_super(bdev);
	if (!sb)
		goto out;

	if (sb->s_flags & MS_RDONLY) {
		sb->s_frozen = SB_FREEZE_TRANS;
		smp_wmb();
		goto out;
	}

	sb->s_frozen = SB_FREEZE_WRITE;
	smp_wmb();

	__fsync_super(sb);

	sb->s_frozen = SB_FREEZE_TRANS
	smp_wmb();

	sync_blockdev(sb->s_bdev);

	if (sb->s_op->write_super_lockfs)
		sb->s_op->write_super_lockfs(sb);

out:

	sync_blockdev(bdev);
	return sb;      /* thaw_bdev releases s->s_umount and bd_mount_sem */
}

2007/10/5, Christoph Hellwig <[email protected]>:

It looks cleaner than mine. I'll resubmit it.

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

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

Because xfs_thaw does nothing for the filesystem whose sb->s_frozen is SB_UNFROZEN. So frozen read-only XFS filesystem will never be unfrozen. Thus 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.

(*) freezing read-only filesystem is not so pointless. Because it can prevent from someone trying to remount read/write while freezing.

---
 fs/buffer.c |   25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

Index: 2.6-git/fs/buffer.c
===================================================================
@@ -190,21 +190,28 @@ struct super_block *freeze_bdev(struct b
 
 	down(&bdev->bd_mount_sem);
 	sb = get_super(bdev);
-	if (sb && !(sb->s_flags & MS_RDONLY)) {
-		sb->s_frozen = SB_FREEZE_WRITE;
-		smp_wmb();
-
-		__fsync_super(sb);
+	if (!sb)
+		goto out;
 
+	if (sb->s_flags & MS_RDONLY) {
 		sb->s_frozen = SB_FREEZE_TRANS;
 		smp_wmb();
+		goto out;
+	}
 
-		sync_blockdev(sb->s_bdev);
+	sb->s_frozen = SB_FREEZE_WRITE;
+	smp_wmb();
 
-		if (sb->s_op->write_super_lockfs)
-			sb->s_op->write_super_lockfs(sb);
-	}
+	__fsync_super(sb);
+
+	sb->s_frozen = SB_FREEZE_TRANS;
+	smp_wmb();
 
+	sync_blockdev(sb->s_bdev);
+
+	if (sb->s_op->write_super_lockfs)
+		sb->s_op->write_super_lockfs(sb);
+out:
 	sync_blockdev(bdev);
 	return sb;	/* thaw_bdev releases s->s_umount and bd_mount_sem */
 }