netdev - Re: [RFC Patch net-next 5/6] net_sched: use rcu in fast path

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Date:   Tue, 06 Sep 2016 07:52:08 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, jhs@...atatu.com
Subject: Re: [RFC Patch net-next 5/6] net_sched: use rcu in fast path

On Thu, 2016-09-01 at 22:57 -0700, Cong Wang wrote:



Missing changelog ?

Here I have no idea what you want to fix, since John already took care
all this infra.

Adding extra rcu_dereference() and rcu_read_lock() while the critical
RCU dereferences already happen in callers is not needed.

> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/sched/act_api.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sched/act_api.c b/net/sched/act_api.c
> index 2f8db3c..fb6ff52 100644
> --- a/net/sched/act_api.c
> +++ b/net/sched/act_api.c
> @@ -470,10 +470,14 @@ int tcf_action_exec(struct sk_buff *skb, struct tc_action **actions,
>  		goto exec_done;
>  	}
>  	for (i = 0; i < nr_actions; i++) {
> -		const struct tc_action *a = actions[i];
> +		const struct tc_action *a;
>  
> +		rcu_read_lock();

But the caller already has rcu_read_lock() or rcu_read_lock_bh()

This was done in commit 25d8c0d55f241ce2 ("net: rcu-ify tcf_proto")

> +		a = rcu_dereference(actions[i]);


Add in your .config :
CONFIG_SPARSE_RCU_POINTER=y
make C=2 M=net/sched

>  repeat:
>  		ret = a->ops->act(skb, a, res);
> +		rcu_read_unlock();
> +
>  		if (ret == TC_ACT_REPEAT)
>  			goto repeat;	/* we need a ttl - JHS */
>  		if (ret != TC_ACT_PIPE)



I do not believe this patch is necessary.

Please add John as reviewer next time.

Thanks.