From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 30 Nov 2007 16:37:21 -0800 From: "Paul E. McKenney" Message-ID: <20071201003721.GA22726@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Subject: [Bridge] [PATCH] Remove rcu_assign_pointer() penalty for NULL pointers Reply-To: paulmck@linux.vnet.ibm.com List-Id: Linux Ethernet Bridging List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: linux-kernel@vger.kernel.org Cc: herbert@gondor.apana.org.au, ego@in.ibm.com, netdev@vger.kernel.org, dipankar@in.ibm.com, akpm@linux-foundation.org, bridge@lists.osdl.org, devel@openvz.org, xemul@openvz.org Hello! 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 --- rcupdate.h | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff -urpNa -X dontdiff linux-2.6.24-rc1-ego/include/linux/rcupdate.h linux-2.6.24-rc1-egoxu/include/linux/rcupdate.h --- linux-2.6.24-rc1-ego/include/linux/rcupdate.h 2007-11-06 15:30:02.000000000 -0800 +++ linux-2.6.24-rc1-egoxu/include/linux/rcupdate.h 2007-11-30 09:06:11.000000000 -0800 @@ -191,10 +191,13 @@ static inline void rcu_preempt_boost(voi * code. */ -#define rcu_assign_pointer(p, v) ({ \ - smp_wmb(); \ - (p) = (v); \ - }) +#define rcu_assign_pointer(p, v) \ + ({ \ + if (!__builtin_constant_p(v) || \ + ((v) != NULL)) \ + smp_wmb(); \ + (p) = (v); \ + }) /** * synchronize_sched - block until all CPUs have exited any non-preemptive