Saner locking around deactivate_super()

Make sure that s_umount is acquired *before* we drop the final
active reference; we still have the fast path (atomic_dec_unless)
and we have gotten rid of the window between the moment when
s_active hits zero and s_umount is acquired.  Which simplifies
the living hell out of grab_super() and inotify pin_to_kill()
stuff.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 19 insertions, 26 deletions

diff --git a/fs/super.c b/fs/super.c
index bc734f8b3e18..157657b32798 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -178,53 +178,48 @@ void put_super(struct super_block *sb)
 
 
 /**
- *	deactivate_super	-	drop an active reference to superblock
+ *	deactivate_locked_super	-	drop an active reference to superblock
  *	@s: superblock to deactivate
  *
- *	Drops an active reference to superblock, acquiring a temprory one if
- *	there is no active references left.  In that case we lock superblock,
+ *	Drops an active reference to superblock, converting it into a temprory
+ *	one if there is no other active references left.  In that case we
  *	tell fs driver to shut it down and drop the temporary reference we
  *	had just acquired.
+ *
+ *	Caller holds exclusive lock on superblock; that lock is released.
  */
-void deactivate_super(struct super_block *s)
+void deactivate_locked_super(struct super_block *s)
 {
 	struct file_system_type *fs = s->s_type;
 	if (atomic_dec_and_test(&s->s_active)) {
 		vfs_dq_off(s, 0);
-		down_write(&s->s_umount);
 		fs->kill_sb(s);
 		put_filesystem(fs);
 		put_super(s);
+	} else {
+		up_write(&s->s_umount);
 	}
 }
 
-EXPORT_SYMBOL(deactivate_super);
+EXPORT_SYMBOL(deactivate_locked_super);
 
 /**
- *	deactivate_locked_super	-	drop an active reference to superblock
+ *	deactivate_super	-	drop an active reference to superblock
  *	@s: superblock to deactivate
  *
- *	Equivalent of up_write(&s->s_umount); deactivate_super(s);, except that
- *	it does not unlock it until it's all over.  As the result, it's safe to
- *	use to dispose of new superblock on ->get_sb() failure exits - nobody
- *	will see the sucker until it's all over.  Equivalent using up_write +
- *	deactivate_super is safe for that purpose only if superblock is either
- *	safe to use or has NULL ->s_root when we unlock.
+ *	Variant of deactivate_locked_super(), except that superblock is *not*
+ *	locked by caller.  If we are going to drop the final active reference,
+ *	lock will be acquired prior to that.
  */
-void deactivate_locked_super(struct super_block *s)
+void deactivate_super(struct super_block *s)
 {
-	struct file_system_type *fs = s->s_type;
-	if (atomic_dec_and_test(&s->s_active)) {
-		vfs_dq_off(s, 0);
-		fs->kill_sb(s);
-		put_filesystem(fs);
-		put_super(s);
-	} else {
-		up_write(&s->s_umount);
+        if (!atomic_add_unless(&s->s_active, -1, 1)) {
+		down_write(&s->s_umount);
+		deactivate_locked_super(s);
 	}
 }
 
-EXPORT_SYMBOL(deactivate_locked_super);
+EXPORT_SYMBOL(deactivate_super);
 
 /**
  *	grab_super - acquire an active reference
@@ -247,12 +242,10 @@ static int grab_super(struct super_block *s) __releases(sb_lock)
 	/* it's going away */
 	s->s_count++;
 	spin_unlock(&sb_lock);
-	/* usually that'll be enough for it to die... */
+	/* wait for it to die */
 	down_write(&s->s_umount);
 	up_write(&s->s_umount);
 	put_super(s);
-	/* ... but in case it wasn't, let's at least yield() */
-	yield();
 	return 0;
 }