linux-kernel - Re: [PATCH] cls_cgroup: read classid atomically in classifier

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Date:	Thu, 21 May 2009 08:49:10 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Paul Menage <menage@...gle.com>
CC:	akpm@...ux-foundation.org, davem@...emloft.net, tgraf@...g.ch,
	linux-kernel@...r.kernel.org, linux-net@...r.kernel.org
Subject: Re: [PATCH] cls_cgroup: read classid atomically in classifier

(better CC netdev)

Paul Menage wrote:
> cls_cgroup: read classid atomically in classifier
> 
> Avoid reading the unsynchronized value cs->classid multiple times,
> since it could change concurrently from non-zero to zero; this would
> result in the classifier returning a positive result with a bogus
> (zero) classid.
> 

This patch looks nice. It also narrows rcu read section.

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

> Signed-off-by: Paul Menage <menage@...gle.com>
> 
> ---
> 
>  net/sched/cls_cgroup.c |   22 +++++++++++-----------
>  1 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
> index 1ab4542..4ece6e0 100644
> --- a/net/sched/cls_cgroup.c
> +++ b/net/sched/cls_cgroup.c
> @@ -98,8 +98,7 @@ static int cls_cgroup_classify(struct sk_buff *skb, struct tcf_proto *tp,
>  			       struct tcf_result *res)
>  {
>  	struct cls_cgroup_head *head = tp->root;
> -	struct cgroup_cls_state *cs;
> -	int ret = 0;
> +        u32 classid;
>  
>  	/*
>  	 * Due to the nature of the classifier it is required to ignore all
> @@ -115,17 +114,18 @@ static int cls_cgroup_classify(struct sk_buff *skb, struct tcf_proto *tp,
>  		return -1;
>  
>  	rcu_read_lock();
> -	cs = task_cls_state(current);
> -	if (cs->classid && tcf_em_tree_match(skb, &head->ematches, NULL)) {
> -		res->classid = cs->classid;
> -		res->class = 0;
> -		ret = tcf_exts_exec(skb, &head->exts, res);
> -	} else
> -		ret = -1;
> -
> +	classid = task_cls_state(current)->classid;
>  	rcu_read_unlock();
>  
> -	return ret;
> +	if (!classid)
> +		return -1;
> +
> +	if (!tcf_em_tree_match(skb, &head->ematches, NULL))
> +		return -1;
> +
> +	res->classid = classid;
> +	res->class = 0;
> +	return tcf_exts_exec(skb, &head->exts, res);
>  }
>  
>  static unsigned long cls_cgroup_get(struct tcf_proto *tp, u32 handle)
> 
> 
> 
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