lookup_open(): lock the parent shared unless O_CREAT is given

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

Documentation/filesystems/porting

@@ -575,3 +575,6 @@ in your dentry operations instead.
 
 	Old method is only used if the new one is absent; eventually it will
 	be removed.  Switch while you still can; the old one won't stay.
+--
+[mandatory]
+	->atomic_open() calls without O_CREAT may happen in parallel.

fs/namei.c

@@ -3084,7 +3084,7 @@ static int do_last(struct nameidata *nd,
 	}
 
 retry_lookup:
-	if (op->open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
+	if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
 		error = mnt_want_write(nd->path.mnt);
 		if (!error)
 			got_write = true;
@@ -3094,9 +3094,15 @@ static int do_last(struct nameidata *nd,
 		 * dropping this one anyway.
 		 */
 	}
+	if (open_flag & O_CREAT)
 		inode_lock(dir->d_inode);
+	else
+		inode_lock_shared(dir->d_inode);
 	error = lookup_open(nd, &path, file, op, got_write, opened);
+	if (open_flag & O_CREAT)
 		inode_unlock(dir->d_inode);
+	else
+		inode_unlock_shared(dir->d_inode);
 
 	if (error <= 0) {
 		if (error)