[RFC 2/9] fsfreeze: add unlocked version of thaw_super

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

thaw_super may be called with superblock lock already taken (fsfreeze's
emergency thaw being one example), so we need an unlocked version to avoid
lockups.

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

diff -urNp linux-3.6-rc5-orig/fs/super.c linux-3.6-rc5/fs/super.c
--- linux-3.6-rc5-orig/fs/super.c	2012-09-12 19:13:00.710047001 +0900
+++ linux-3.6-rc5/fs/super.c	2012-09-12 19:13:12.185924077 +0900
@@ -1420,40 +1420,58 @@ int freeze_super(struct super_block *sb)
 EXPORT_SYMBOL(freeze_super);
 
 /**
- * thaw_super -- unlock filesystem
+ * __thaw_super -- unlock filesystem
  * @sb: the super to thaw
  *
- * Unlocks the filesystem and marks it writeable again after freeze_super().
+ * Unlocks the filesystem and marks it writeable again.
+ *
+ * This is the unlocked version of thaw_super, so it is the caller's job to
+ * to protect the superblock by grabing the s_umount semaphore in write mode.
  */
-int thaw_super(struct super_block *sb)
+int __thaw_super(struct super_block *sb)
 {
-	int error;
+	int error = 0;
 
-	down_write(&sb->s_umount);
 	if (sb->s_writers.frozen == SB_UNFROZEN) {
-		up_write(&sb->s_umount);
-		return -EINVAL;
+		error = -EINVAL;
+		goto out;
 	}
 
 	if (sb->s_flags & MS_RDONLY)
-		goto out;
+		goto out_thaw;
 
 	if (sb->s_op->unfreeze_fs) {
 		error = sb->s_op->unfreeze_fs(sb);
 		if (error) {
 			printk(KERN_ERR
 				"VFS:Filesystem thaw failed\n");
-			up_write(&sb->s_umount);
-			return error;
+			goto out;
 		}
 	}
 
-out:
+out_thaw:
 	sb->s_writers.frozen = SB_UNFROZEN;
 	smp_wmb();
 	wake_up(&sb->s_writers.wait_unfrozen);
-	deactivate_locked_super(sb);
+out:
+	return error;
+}
 
-	return 0;
+/**
+ * thaw_super -- unlock filesystem
+ * @sb: the super to thaw
+ *
+ * Unlocks the filesystem and marks it writeable again after freeze_super().
+ */
+int thaw_super(struct super_block *sb)
+{
+	int res;
+	down_write(&sb->s_umount);
+	res = __thaw_super(sb);
+	if (!res)
+		deactivate_locked_super(sb);
+	else
+		up_write(&sb->s_umount);
+	return res;
 }
 EXPORT_SYMBOL(thaw_super);
diff -urNp linux-3.6-rc5-orig/include/linux/fs.h linux-3.6-rc5/include/linux/fs.h
--- linux-3.6-rc5-orig/include/linux/fs.h	2012-09-12 19:13:00.830047035 +0900
+++ linux-3.6-rc5/include/linux/fs.h	2012-09-12 19:13:12.197929820 +0900
@@ -2082,6 +2082,7 @@ extern int user_statfs(const char __user
 extern int fd_statfs(int, struct kstatfs *);
 extern int vfs_ustat(dev_t, struct kstatfs *);
 extern int freeze_super(struct super_block *super);
+extern int __thaw_super(struct super_block *super);
 extern int thaw_super(struct super_block *super);
 extern bool our_mnt(struct vfsmount *mnt);