linux-kernel - Re: [PATCH] cpuset: rcu_read_lock() to protect task_cs() even we don't dereference to task

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Message-ID: <492775ED.1040803@cn.fujitsu.com>
Date:	Sat, 22 Nov 2008 11:01:01 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Paul Menage <menage@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH] cpuset: rcu_read_lock() to protect task_cs() even we
 	don't dereference to task_cs()'s return value

Paul Menage wrote:
> On Fri, Nov 21, 2008 at 11:05 AM, Paul Menage <menage@...gle.com> wrote:
>> Wouldn't this be fixed by your patch that adds an rcu_read_lock() to
>> task_subsys_state() ?
>>
> 
> Sorry, your other patch actually adds the rcu_read_lock() to
> task_cgroup(). But adding an rcu_read_lock() to task_subsys_state()
> would also help here.

Adding an rcu_read_lock() to task_subsys_state() also help here.
But task_subsys_state() is used in fast path.

If we add an implicit rcu_read_lock() in task_subsys_state(),
we still need rcu_read_lock()/task_lock() for using it,
so it's redundant rcu_read_lock(), and slower the fast path a little.

Lai.

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