linux-kernel - Re: INFO: suspicious rcu_dereference_check() usage - include/linux/cgroup.h:492 invoked rcu_dereference

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Message-ID: <4BCE4F6E.10301@cn.fujitsu.com>
Date:	Wed, 21 Apr 2010 09:05:50 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Eric Paris <eparis@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Paris <eparis@...isplace.org>,
	Miles Lane <miles.lane@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: INFO: suspicious rcu_dereference_check() usage - include/linux/cgroup.h:492
 invoked rcu_dereference_check() without protection!

> commit 2836f18139267ea918ed2cf39023fb0eb38c4361
> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Date:   Mon Apr 19 15:59:50 2010 -0700
> 
>     rcu: fix RCU lockdep splat on freezer_fork path
>     
>     Add an RCU read-side critical section to suppress this false positive.
>     
>     Located-by: Eric Paris <eparis@...isplace.org>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> index da5e139..e5c0244 100644
> --- a/kernel/cgroup_freezer.c
> +++ b/kernel/cgroup_freezer.c
> @@ -205,9 +205,12 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
>  	 * No lock is needed, since the task isn't on tasklist yet,
>  	 * so it can't be moved to another cgroup, which means the
>  	 * freezer won't be removed and will be valid during this
> -	 * function call.
> +	 * function call.  Nevertheless, apply RCU read-side critical
> +	 * section to suppress RCU lockdep false positives.
>  	 */

It was me that added the original comment.

> +	rcu_read_lock();
>  	freezer = task_freezer(task);
> +	rcu_read_unlock();
>  
>  	/*
>  	 * The root cgroup is non-freezable, so we can skip the

Acked-by: Li Zefan <lizf@...fujitsu.com>
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