linux-kernel - Re: [PATCH 05/17] cgroup: cgroup->dentry isn't a RCU pointer

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Date:	Wed, 14 Nov 2012 15:05:46 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<lizefan@...wei.com>, <containers@...ts.linux-foundation.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<mhocko@...e.cz>
Subject: Re: [PATCH 05/17] cgroup: cgroup->dentry isn't a RCU pointer

On 11/13/2012 07:01 AM, Tejun Heo wrote:
> cgroup->dentry is marked and used as a RCU pointer; however, it isn't
> one - the final dentry put doesn't go through call_rcu().  cgroup and
> dentry share the same RCU freeing rule via synchronize_rcu() in
> cgroup_diput() (kfree_rcu() used on cgrp is unnecessary).  If cgrp is
> accessible under RCU read lock, so is its dentry and dereferencing
> cgrp->dentry doesn't need any further RCU protection or annotation.
> 
> While not being accurate, before the previous patch, the RCU accessors
> served a purpose as memory barriers - cgroup->dentry used to be
> assigned after the cgroup was made visible to cgroup_path(), so the
> assignment and dereferencing in cgroup_path() needed the memory
> barrier pair.  Now that list_add_tail_rcu() happens after
> cgroup->dentry is assigned, this no longer is necessary.
> 
> Remove the now unnecessary and misleading RCU annotations from
> cgroup->dentry.  To make up for the removal of rcu_dereference_check()
> in cgroup_path(), add an explicit rcu_lockdep_assert(), which asserts
> the dereference rule of @cgrp, not cgrp->dentry.
> 

Will memcontrol.c need similar amendments?

The code that lives in -mm and includes kmemcg includes the following
excerpt:

        rcu_read_lock();
        dentry = rcu_dereference(memcg->css.cgroup->dentry);
        rcu_read_unlock();


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