linux-kernel - Re: [PATCH 04/17] cgroup: create directory before linking while creating a new cgroup

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Date:	Fri, 16 Nov 2012 14:04:29 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<containers@...ts.linux-foundation.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <mhocko@...e.cz>,
	<glommer@...allels.com>
Subject: Re: [PATCH 04/17] cgroup: create directory before linking while creating
 a new cgroup

On 2012/11/15 3:04, Tejun Heo wrote:
> Hello, Li.
> 
> On Wed, Nov 14, 2012 at 11:20:47AM +0800, Li Zefan wrote:
>>> It also removes the need to check whether cgroup->dentry is %NULL in
>>> cgroup_path.  If a cgroup is visible, its dentry is guaranteed to be
>>> visible too.
>>
>> I'm afraid this isn't safe.
>>
>> The cgroup is visible to a controller soon after ss->create(), and then
>> the controller might call cgroup_path() while cgrp->dentry is still NULL.
> 
> Hmmm... I can't find any case where ss->create() is calling
> cgroup_path().  Do you remember which one that was?
> 

I meant cgroup_create() may race with cgroup_path(). We used to have
this race:

(cpu0)                          (cpu1)
mkdir                           cat /proc/sched_debug

cgroup_create()
  cpu_cgroup_create()
    register_fair_sched_group()
      list_add_rcu(...)
				print_cfs_stats()
				  for_each_leaf_cfs_rq()
				    print_cfs_rq()
				      cgroup_path()
  css->cgroup = cgroup;
  cgroup->dentry = dentry;

In this case, cgroup_path() might access NULL cgroup or NULL dentry.

But I've looked the current scheduler code, no race anymore, because now the
cfs_rq won't be added to the gloal list until there's task in the cgroup.
(in the old time it was added to the list when creating a cgroup)

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