vfs: Provide function to get superblock and wait for it to thaw

In quota code we need to find a superblock corresponding to a device and wait
for superblock to be unfrozen. However this waiting has to happen without
s_umount semaphore because that is required for superblock to thaw. So provide
a function in VFS for this to keep dances with s_umount where they belong.

[AV: implementation switched to saner variant]
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/super.c

@@ -633,6 +633,28 @@ struct super_block *get_super(struct block_device *bdev)
 
 EXPORT_SYMBOL(get_super);
 
+/**
+ *	get_super_thawed - get thawed superblock of a device
+ *	@bdev: device to get the superblock for
+ *
+ *	Scans the superblock list and finds the superblock of the file system
+ *	mounted on the device. The superblock is returned once it is thawed
+ *	(or immediately if it was not frozen). %NULL is returned if no match
+ *	is found.
+ */
+struct super_block *get_super_thawed(struct block_device *bdev)
+{
+	while (1) {
+		struct super_block *s = get_super(bdev);
+		if (!s || s->s_frozen == SB_UNFROZEN)
+			return s;
+		up_read(&s->s_umount);
+		vfs_check_frozen(s, SB_FREEZE_WRITE);
+		put_super(s);
+	}
+}
+EXPORT_SYMBOL(get_super_thawed);
+
 /**
  * get_active_super - get an active reference to the superblock of a device
  * @bdev: device to get the superblock for

include/linux/fs.h

@@ -2496,6 +2496,7 @@ extern void get_filesystem(struct file_system_type *fs);
 extern void put_filesystem(struct file_system_type *fs);
 extern struct file_system_type *get_fs_type(const char *name);
 extern struct super_block *get_super(struct block_device *);
+extern struct super_block *get_super_thawed(struct block_device *);
 extern struct super_block *get_active_super(struct block_device *bdev);
 extern void drop_super(struct super_block *sb);
 extern void iterate_supers(void (*)(struct super_block *, void *), void *);