linux-kernel - Re: [PATCH 6/6] cls_cgroup: remove redundant rcu_read

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Date:	Wed, 01 Feb 2012 08:07:19 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>,
	Neil Horman <nhorman@...driver.com>
Subject: Re: [PATCH 6/6] cls_cgroup: remove redundant rcu_read_lock/unlock

Le mercredi 01 février 2012 à 14:56 +0800, Li Zefan a écrit :
> We've already used rcu_read_lock/unlock inside task_classid(),
> so don't use the lock/unlock pair twice in this hot path.
> 
> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> ---
>  net/core/sock.c |    2 --
>  1 files changed, 0 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 213c856..c0bab23 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1160,9 +1160,7 @@ void sock_update_classid(struct sock *sk)
>  {
>  	u32 classid;
>  
> -	rcu_read_lock();  /* doing current task, which cannot vanish. */
>  	classid = task_cls_classid(current);
> -	rcu_read_unlock();
>  	if (classid && classid != sk->sk_classid)
>  		sk->sk_classid = classid;

Yes, this seems fine.

Then, I wonder why we do the "if (classid && classid != sk->sk_classid)"

before the :

	sk->sk_classid = classid;

This seems unnecessary checks.


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