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

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Message-ID: <20120201072307.GA6358@gondor.apana.org.au>
Date:	Wed, 1 Feb 2012 18:23:07 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	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

On Wed, Feb 01, 2012 at 03:20:00PM +0800, Li Zefan wrote:
> Cc: Herbert Xu <herbert@...dor.apana.org.au>
> 
> >> 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.
> > 
> 
> I was wondering about this too. He who added this may provide us with an
> answer.

Well writing a cache-line unnecessarily is bad.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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