From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752894AbaIXTZC (ORCPT ); Wed, 24 Sep 2014 15:25:02 -0400 Received: from mail-qa0-f47.google.com ([209.85.216.47]:55132 "EHLO mail-qa0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751182AbaIXTZA (ORCPT ); Wed, 24 Sep 2014 15:25:00 -0400 Date: Wed, 24 Sep 2014 15:24:56 -0400 From: Tejun Heo To: Al Viro Cc: Andrey Wagin , LKML , Li Zefan Subject: Re: linux-next: cgroup_mount() falls asleep forever Message-ID: <20140924192456.GA1233@mtj.dyndns.org> References: <20140924185213.GB7996@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140924185213.GB7996@ZenIV.linux.org.uk> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hey, Al. On Wed, Sep 24, 2014 at 07:52:14PM +0100, Al Viro wrote: > On Wed, Sep 24, 2014 at 06:29:27PM +0400, Andrey Wagin wrote: > > 2014-09-24 14:31 GMT+04:00 Andrey Wagin : > > > Hi All, > > > > The problem is in a following commit: > > > > commit 0c7bf3e8cab7900e17ce7f97104c39927d835469 > > Author: Zefan Li > > Date: Sat Sep 20 14:49:10 2014 +0800 > > > > cgroup: remove redundant variable in cgroup_mount() > > > > Both pinned_sb and new_sb indicate if a new superblock is needed, > > so we can just remove new_sb. > > > > Note now we must check if kernfs_tryget_sb() returns NULL, because > > when it returns NULL, kernfs_mount() may still re-use an existing > > superblock, which is just allocated by another concurent mount. > > > > Suggested-by: Tejun Heo > > Signed-off-by: Zefan Li > > Signed-off-by: Tejun Heo I'm gonna wait for Li's response for a couple days and then revert it if it can't be fixed differently. > Lovely... First of all, that thing is obviously racy - there's nothing > to prevent another mount happening between these two places. Moreover, > kernfs_mount() calling conventions are really atrocious - pointer to > bool just to indicate that superblock is new? > > Could somebody explain WTF is the whole construction trying to do? Not > to mention anything else, what *does* this pinning a superblock protect > from? Suppose we have a superblock for the same root with non-NULL ns > and _that_ gets killed. We get hit by the same > percpu_ref_kill(&root->cgrp.self.refcnt); > so what's the point of pinned_sb? Might as well have just bumped the > refcount, superblock or no superblock. And no, delaying that kernfs_kill_sb() > does you no good whatsoever - again, pinned_sb might have nothing to do with > the superblock we are after. Yeah, it's an ugly thing to work around vfs interface not very conducive for filesystems which conditionally create or reuse superblocks during mount. There was a thread explaining what's going on. Looking up... http://thread.gmane.org/gmane.linux.kernel.containers/27623/focus=10635 Thanks. -- tejun