From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Fri, 30 Nov 2007 22:00:58 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Message-ID: <20071201060058.GA4734@linux.vnet.ibm.com>
References: <20071201003721.GA22726@linux.vnet.ibm.com>
	<20071201010752.GA26621@gondor.apana.org.au>
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Subject: [Bridge] Re: [PATCH] Remove rcu_assign_pointer() penalty for NULL
	pointers
Reply-To: paulmck@linux.vnet.ibm.com
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To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: ego@in.ibm.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, dipankar@in.ibm.com, akpm@linux-foundation.org, bridge@lists.osdl.org, devel@openvz.org, xemul@openvz.org

On Sat, Dec 01, 2007 at 12:07:52PM +1100, Herbert Xu wrote:
> 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@linux.vnet.ibm.com>
> 
> Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
> 
> Thanks a lot for following through with this Paul!

No problem -- after all, it is not every day that one gets the opportunity
to make a simple change that speeds things up and makes kernel hackers
lives a bit simpler.  ;-)

						Thanx, Paul