From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758922Ab3GZNjE (ORCPT ); Fri, 26 Jul 2013 09:39:04 -0400 Received: from mail-ye0-f180.google.com ([209.85.213.180]:46722 "EHLO mail-ye0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755189Ab3GZNjB (ORCPT ); Fri, 26 Jul 2013 09:39:01 -0400 Date: Fri, 26 Jul 2013 09:38:56 -0400 From: Tejun Heo To: Dennis Chen Cc: linux-kernel@vger.kernel.org, gregkh@linuxfoundation.org, xiyou.wangcong@gmail.com Subject: Re: [PATCH] race condition fixing in sysfs_create_dir Message-ID: <20130726133856.GD3013@htj.dyndns.org> References: <51F24612.6030509@tnsoft.com.cn> <51F24864.7070306@tnsoft.com.cn> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <51F24864.7070306@tnsoft.com.cn> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, On Fri, Jul 26, 2013 at 05:59:00PM +0800, Dennis Chen wrote: > On 07/26/2013 05:49 PM, Dennis Chen wrote: > > >The patch is trying its best to avoid creating a dir under a parent dir which is removing from > >the system: > > PATH0 (create a dir under 'PARENT/...') PATH1 (remove the 'PARENT/...') > > sysfs_create_dir() { sysfs_remove_dir() { > > ... ... > > if (kobj->parent) spin_lock(&sysfs_assoc_lock); > > parent_sd = kobj->parent->sd; <----- kobj->sd = NULL; > > else spin_unlock(&sysfs_assoc_lock); > > parent_sd = &sysfs_root; > >Suppose PATH1 enter the critical section first, then PATH0 begin to execute before kobj->sd > >has been reset to NULL, possibly PATH0 will get a non-NULL parent_sd since lack of the > >sysfs_assoc_lock protection in PATH0. In this case, PATH0 think it has a valid parent_sd which > >can be freed by PATH1 in the followed, refer to the comments in the patch. Maybe we need > >to figure out a perfect solution to solve the race condition, although the codes in question are > >in slow path... I don't think sysfs is supposed to handle multiple actors trying to populate and destroy the directory at the same time at all, so this seems kinda moot. Do you have a case where this actually matters? Thanks. -- tejun