From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755078Ab1IHXb4 (ORCPT ); Thu, 8 Sep 2011 19:31:56 -0400 Received: from mx1.redhat.com ([209.132.183.28]:46129 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755166Ab1IHXbx (ORCPT ); Thu, 8 Sep 2011 19:31:53 -0400 Date: Thu, 8 Sep 2011 19:35:59 +0200 From: Oleg Nesterov To: Ben Blum Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, fweisbec@gmail.com, neilb@suse.de, paul@paulmenage.org, paulmck@linux.vnet.ibm.com Subject: Re: + cgroups-more-safe-tasklist-locking-in-cgroup_attach_proc.patch added to -mm tree Message-ID: <20110908173559.GA26492@redhat.com> References: <201109012108.p81L8X0b029484@imap1.linux-foundation.org> <20110902123706.GB26764@redhat.com> <20110902140015.GA31530@redhat.com> <20110902141550.GA24012@unix33.andrew.cmu.edu> <20110902155534.GA4595@redhat.com> <20110907235931.GA22545@unix33.andrew.cmu.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110907235931.GA22545@unix33.andrew.cmu.edu> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/07, Ben Blum wrote: > > On Fri, Sep 02, 2011 at 05:55:34PM +0200, Oleg Nesterov wrote: > > On 09/02, Ben Blum wrote: > > > > > > But I don't think the check becomes pointless? If a sub-thread execs > > > right before read_lock(&tasklist_lock) (but after the find_task_by_vpid > > > in attach_task_by_pid), that causes the case that the comment refers to. > > > > How so? The comment says: > > > > * a race with de_thread from another thread's exec() may strip > > * us of our leadership, making while_each_thread unsafe > > > > This is not true. > > Sorry, the comment is unclear. No, the comment is clear. In fact it was me who pointed out we can't do while_each_thread() blindly. And now I am tried to confuse you ;) So, sorry for noise, and thanks for correcting me. Somehow I forgot this is not safe even under tasklist. Partly I was confused because I was thinking about the patch I suggested, if we use ->siglock we are safe. If lock_task_sighand(task) succeeds, this task should be on list. Anyway, I was wrong, sorry. Oleg. --- x/kernel/cgroup.c +++ x/kernel/cgroup.c @@ -2000,6 +2000,7 @@ int cgroup_attach_proc(struct cgroup *cg /* threadgroup list cursor and array */ struct task_struct *tsk; struct flex_array *group; + unsigned long flags; /* * we need to make sure we have css_sets for all the tasks we're * going to move -before- we actually start moving them, so that in @@ -2027,19 +2028,10 @@ int cgroup_attach_proc(struct cgroup *cg goto out_free_group_list; /* prevent changes to the threadgroup list while we take a snapshot. */ - rcu_read_lock(); - if (!thread_group_leader(leader)) { - /* - * a race with de_thread from another thread's exec() may strip - * us of our leadership, making while_each_thread unsafe to use - * on this task. if this happens, there is no choice but to - * throw this task away and try again (from cgroup_procs_write); - * this is "double-double-toil-and-trouble-check locking". - */ - rcu_read_unlock(); - retval = -EAGAIN; + retval = -EAGAIN; + if (!lock_task_sighand(leader, &flags)) goto out_free_group_list; - } + /* take a reference on each task in the group to go in the array. */ tsk = leader; i = 0; @@ -2055,9 +2047,9 @@ int cgroup_attach_proc(struct cgroup *cg BUG_ON(retval != 0); i++; } while_each_thread(leader, tsk); + unlock_task_sighand(leader, &flags); /* remember the number of threads in the array for later. */ group_size = i; - rcu_read_unlock(); /* * step 1: check that we can legitimately attach to the cgroup.