[PATCH 1/3] fs: Add activate_super function

[Andreas Gruenbacher <agruenba@redhat.com>]

Add function activate_super() for grabbing an active reference on a
super block.  Fails if the filesystem is already shutting down.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/super.c         | 19 +++++++++++++++++++
 include/linux/fs.h |  1 +
 2 files changed, 20 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 8d39e4f11cfa..051241cf408b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -393,6 +393,25 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
 	return 0;
 }
 
+/**
+ * activate_super - try to grab an active reference on the superblock
+ * @sb: reference we are trying to grab
+ *
+ * Try to grab an active reference on the superblock to prevent filesystem
+ * shutdown.  Fails if the filesystem is already shutting down (see
+ * deactivate_locked_super()).
+ */
+bool activate_super(struct super_block *sb)
+{
+	if (atomic_inc_not_zero(&sb->s_active)) {
+		smp_mb__after_atomic();
+		BUG_ON(!(sb->s_flags & SB_BORN));
+		return true;
+	}
+	return false;
+}
+EXPORT_SYMBOL(activate_super);
+
 /*
  *	trylock_super - try to grab ->s_umount shared
  *	@sb: reference we are trying to grab
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e654435f1651..84c609123a25 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2573,6 +2573,7 @@ void generic_shutdown_super(struct super_block *sb);
 void kill_block_super(struct super_block *sb);
 void kill_anon_super(struct super_block *sb);
 void kill_litter_super(struct super_block *sb);
+bool activate_super(struct super_block *sb);
 void deactivate_super(struct super_block *sb);
 void deactivate_locked_super(struct super_block *sb);
 int set_anon_super(struct super_block *s, void *data);
-- 
2.38.1