massage __follow_mount_rcu() a bit

make the loop more similar to that in follow_managed(), with
explicit tracking of flags, etc.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1269,12 +1269,6 @@ int follow_down_one(struct path *path)
 }
 EXPORT_SYMBOL(follow_down_one);
 
-static inline int managed_dentry_rcu(const struct path *path)
-{
-	return (path->dentry->d_flags & DCACHE_MANAGE_TRANSIT) ?
-		path->dentry->d_op->d_manage(path, true) : 0;
-}
-
 /*
  * Try to skip to top of mountpoint pile in rcuwalk mode.  Fail if
  * we meet a managed dentry that would need blocking.
@@ -1282,43 +1276,49 @@ static inline int managed_dentry_rcu(const struct path *path)
 static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 			       struct inode **inode, unsigned *seqp)
 {
+	struct dentry *dentry = path->dentry;
+	unsigned int flags = dentry->d_flags;
+
+	if (likely(!(flags & DCACHE_MANAGED_DENTRY)))
+		return true;
+
+	if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+		return false;
+
 	for (;;) {
-		struct mount *mounted;
 		/*
 		 * Don't forget we might have a non-mountpoint managed dentry
 		 * that wants to block transit.
 		 */
-		switch (managed_dentry_rcu(path)) {
-		case -ECHILD:
-		default:
-			return false;
-		case -EISDIR:
-			return true;
-		case 0:
-			break;
+		if (unlikely(flags & DCACHE_MANAGE_TRANSIT)) {
+			int res = dentry->d_op->d_manage(path, true);
+			if (res)
+				return res == -EISDIR;
+			flags = dentry->d_flags;
 		}
 
-		if (!d_mountpoint(path->dentry))
-			return !(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
-
-		mounted = __lookup_mnt(path->mnt, path->dentry);
-		if (!mounted)
-			break;
-		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
-			return false;
+		if (flags & DCACHE_MOUNTED) {
+			struct mount *mounted = __lookup_mnt(path->mnt, dentry);
+			if (mounted) {
 				path->mnt = &mounted->mnt;
-		path->dentry = mounted->mnt.mnt_root;
+				dentry = path->dentry = mounted->mnt.mnt_root;
 				nd->flags |= LOOKUP_JUMPED;
-		*seqp = read_seqcount_begin(&path->dentry->d_seq);
+				*seqp = read_seqcount_begin(&dentry->d_seq);
+				*inode = dentry->d_inode;
 				/*
-		 * Update the inode too. We don't need to re-check the
-		 * dentry sequence number here after this d_inode read,
-		 * because a mount-point is always pinned.
+				 * We don't need to re-check ->d_seq after this
+				 * ->d_inode read - there will be an RCU delay
+				 * between mount hash removal and ->mnt_root
+				 * becoming unpinned.
 				 */
-		*inode = path->dentry->d_inode;
+				flags = dentry->d_flags;
+				continue;
+			}
+			if (read_seqretry(&mount_lock, nd->m_seq))
+				return false;
+		}
+		return !(flags & DCACHE_NEED_AUTOMOUNT);
 	}
-	return !read_seqretry(&mount_lock, nd->m_seq) &&
-		!(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT);
 }
 
 static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,