[PATCH 1/2] freeze_bdev: kill bd_mount_sem

[Christoph Hellwig <hch@lst.de>]

Now that we have the freeze count there is not much reason for bd_mount_sem
anymore.  The actual freeze/thaw operations are serialized using the
bd_fsfreeze_mutex, and the only other place we take bd_mount_sem is
get_sb_bdev which tries to prevent mounting a filesystem while the block
device is frozen.  Instead of add a check for bd_fsfreeze_count and
return -EBUSY if a filesystem is frozen.  While that is a change in user
visible behaviour a failing mount is much better for this case rather
than having the mount process stuck uninterruptible for a long time.

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: linux-2.6/fs/block_dev.c
===================================================================
--- linux-2.6.orig/fs/block_dev.c	2009-08-01 21:05:06.257330315 +0200
+++ linux-2.6/fs/block_dev.c	2009-08-01 21:08:57.094353741 +0200
@@ -216,8 +216,6 @@ EXPORT_SYMBOL(fsync_bdev);
  * freeze_bdev  --  lock a filesystem and force it into a consistent state
  * @bdev:	blockdevice to lock
  *
- * This takes the block device bd_mount_sem to make sure no new mounts
- * happen on bdev until thaw_bdev() is called.
  * If a superblock is found on this device, we take the s_umount semaphore
  * on it to make sure nobody unmounts until the snapshot creation is done.
  * The reference counter (bd_fsfreeze_count) guarantees that only the last
@@ -240,7 +238,6 @@ struct super_block *freeze_bdev(struct b
 	}
 	bdev->bd_fsfreeze_count++;
 
-	down(&bdev->bd_mount_sem);
 	sb = get_super(bdev);
 	if (sb && !(sb->s_flags & MS_RDONLY)) {
 		sb->s_frozen = SB_FREEZE_WRITE;
@@ -260,7 +257,6 @@ struct super_block *freeze_bdev(struct b
 					"VFS:Filesystem freeze failed\n");
 				sb->s_frozen = SB_UNFROZEN;
 				drop_super(sb);
-				up(&bdev->bd_mount_sem);
 				bdev->bd_fsfreeze_count--;
 				mutex_unlock(&bdev->bd_fsfreeze_mutex);
 				return ERR_PTR(error);
@@ -271,7 +267,7 @@ struct super_block *freeze_bdev(struct b
 	sync_blockdev(bdev);
 	mutex_unlock(&bdev->bd_fsfreeze_mutex);
 
-	return sb;	/* thaw_bdev releases s->s_umount and bd_mount_sem */
+	return sb;	/* thaw_bdev releases s->s_umount */
 }
 EXPORT_SYMBOL(freeze_bdev);
 
@@ -321,7 +317,6 @@ int thaw_bdev(struct block_device *bdev,
 		drop_super(sb);
 	}
 
-	up(&bdev->bd_mount_sem);
 	mutex_unlock(&bdev->bd_fsfreeze_mutex);
 	return 0;
 }
@@ -431,7 +426,6 @@ static void init_once(void *foo)
 
 	memset(bdev, 0, sizeof(*bdev));
 	mutex_init(&bdev->bd_mutex);
-	sema_init(&bdev->bd_mount_sem, 1);
 	INIT_LIST_HEAD(&bdev->bd_inodes);
 	INIT_LIST_HEAD(&bdev->bd_list);
 #ifdef CONFIG_SYSFS
Index: linux-2.6/fs/super.c
===================================================================
--- linux-2.6.orig/fs/super.c	2009-08-01 21:05:06.264331137 +0200
+++ linux-2.6/fs/super.c	2009-08-01 21:08:57.094353741 +0200
@@ -740,9 +740,14 @@ int get_sb_bdev(struct file_system_type 
 	 * will protect the lockfs code from trying to start a snapshot
 	 * while we are mounting
 	 */
-	down(&bdev->bd_mount_sem);
+	mutex_lock(&bdev->bd_fsfreeze_mutex);
+	if (bdev->bd_fsfreeze_count > 0) {
+		mutex_unlock(&bdev->bd_fsfreeze_mutex);
+		error = -EBUSY;
+		goto error_bdev;
+	}
 	s = sget(fs_type, test_bdev_super, set_bdev_super, bdev);
-	up(&bdev->bd_mount_sem);
+	mutex_unlock(&bdev->bd_fsfreeze_mutex);
 	if (IS_ERR(s))
 		goto error_s;
 
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h	2009-08-01 21:05:06.271330981 +0200
+++ linux-2.6/include/linux/fs.h	2009-08-01 21:08:57.098355019 +0200
@@ -640,7 +640,6 @@ struct block_device {
 	struct super_block *	bd_super;
 	int			bd_openers;
 	struct mutex		bd_mutex;	/* open/close mutex */
-	struct semaphore	bd_mount_sem;
 	struct list_head	bd_inodes;
 	void *			bd_holder;
 	int			bd_holders;