vfs: fix renameat to retry on ESTALE errors

...as always, rename is the messiest of the bunch. We have to track
whether to retry or not via a separate flag since the error handling
is already quite complex.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -3840,15 +3840,17 @@ SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
 	struct nameidata oldnd, newnd;
 	struct filename *from;
 	struct filename *to;
+	unsigned int lookup_flags = 0;
+	bool should_retry = false;
 	int error;
-
-	from = user_path_parent(olddfd, oldname, &oldnd, 0);
+retry:
+	from = user_path_parent(olddfd, oldname, &oldnd, lookup_flags);
 	if (IS_ERR(from)) {
 		error = PTR_ERR(from);
 		goto exit;
 	}
 
-	to = user_path_parent(newdfd, newname, &newnd, 0);
+	to = user_path_parent(newdfd, newname, &newnd, lookup_flags);
 	if (IS_ERR(to)) {
 		error = PTR_ERR(to);
 		goto exit1;
@@ -3920,11 +3922,18 @@ SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
 	unlock_rename(new_dir, old_dir);
 	mnt_drop_write(oldnd.path.mnt);
 exit2:
+	if (retry_estale(error, lookup_flags))
+		should_retry = true;
 	path_put(&newnd.path);
 	putname(to);
 exit1:
 	path_put(&oldnd.path);
 	putname(from);
+	if (should_retry) {
+		should_retry = false;
+		lookup_flags |= LOOKUP_REVAL;
+		goto retry;
+	}
 exit:
 	return error;
 }