From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Wed, 6 Feb 2008 09:37:25 +0000 (-0800)
Subject: Remove rcu_assign_pointer() penalty for NULL pointers
X-Git-Tag: v2.6.25-rc1~707
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d99c4f6b13b3149bc83703ab1493beaeaaaf8a2d;p=pandora-kernel.git

Remove rcu_assign_pointer() penalty for NULL pointers

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

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