1 files changed, 33 insertions, 0 deletions
diff --git a/fs/super.c b/fs/super.c
index 73ab9f9b357..e63c754447c 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -243,6 +243,39 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
 }
 
 /*
+ *	grab_super_passive - acquire a passive reference
+ *	@s: reference we are trying to grab
+ *
+ *	Tries to acquire a passive reference. This is used in places where we
+ *	cannot take an active reference but we need to ensure that the
+ *	superblock does not go away while we are working on it. It returns
+ *	false if a reference was not gained, and returns true with the s_umount
+ *	lock held in read mode if a reference is gained. On successful return,
+ *	the caller must drop the s_umount lock and the passive reference when
+ *	done.
+ */
+bool grab_super_passive(struct super_block *sb)
+{
+	spin_lock(&sb_lock);
+	if (list_empty(&sb->s_instances)) {
+		spin_unlock(&sb_lock);
+		return false;
+	}
+
+	sb->s_count++;
+	spin_unlock(&sb_lock);
+
+	if (down_read_trylock(&sb->s_umount)) {
+		if (sb->s_root)
+			return true;
+		up_read(&sb->s_umount);
+	}
+
+	put_super(sb);
+	return false;
+}
+
+/*
  * Superblock locking.  We really ought to get rid of these two.
  */
 void lock_super(struct super_block * sb)