linux-kernel - Re: [PATCH 1/2] rcu: add rcu_access_pointer and rcu_dereference

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Message-ID: <1270657757.8141.30.camel@edumazet-laptop>
Date:	Wed, 07 Apr 2010 18:29:17 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Howells <dhowells@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, Trond.Myklebust@...app.com,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rcu: add rcu_access_pointer and
 rcu_dereference_protect

Le mercredi 07 avril 2010 à 17:19 +0100, David Howells a écrit :

> Why not:
> 
> 	ASSERT(atomic_read(&sk->sk_wmem_alloc) == 0);
> 	filter = rcu_dereference(sk->sk_filter);
> 
> This is much clearer, and you're not combining an unrelated assertion with the
> RCU dereference.

1) Because we want the check being done only when CONFIG_PROVE_RCU is
set.

2) Because rcu_dereference() default condition is : 'Am I owning
rcu_read_lock() or equivalent'. 
In this context, I am _not_ owning rcu lock, so we will trigger a
warning.


So this is best done as is :)

I personally find this very clear and clean, this is why I acked Paul
patch :)

If we were 100% sure testing sk_wmem_alloc is not necessary, we would
have put :

filter = rcu_dereference_check(sk->sk_filter, 1);



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