linux-kernel - Re: [PATCH] Remove rcu_assign_pointer() penalty for NULL pointers

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Message-ID: <20071201010752.GA26621@gondor.apana.org.au>
Date:	Sat, 1 Dec 2007 12:07:52 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, ego@...ibm.com,
	netdev@...r.kernel.org, bridge@...ts.osdl.org, devel@...nvz.org,
	shemminger@...ux-foundation.org, xemul@...nvz.org,
	dipankar@...ibm.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] Remove rcu_assign_pointer() penalty for NULL pointers

On Fri, Nov 30, 2007 at 04:37:21PM -0800, Paul E. McKenney wrote:
> 
> The rcu_assign_pointer() primitive currently unconditionally executes
> a memory barrier, even when a NULL pointer is being assigned.  This
> has lead some to avoid using rcu_assign_pointer() for NULL pointers,
> which loses the self-documenting advantages of rcu_assign_pointer()
> This patch uses __builtin_const_p() to omit needless memory barriers
> for NULL-pointer assignments at compile time with no runtime penalty,
> as discussed in the following thread:
> 
> 	http://www.mail-archive.com/netdev@vger.kernel.org/msg54852.html
> 
> Tested on x86_64 and ppc64, also compiled the four cases (NULL/non-NULL
> and const/non-const) with gcc version 4.1.2, and hand-checked the
> assembly output.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Acked-by: Herbert Xu <herbert@...dor.apana.org.au>

Thanks a lot for following through with this Paul!
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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