From: Paul E. McKenney 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: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d99c4f6b13b3149bc83703ab1493beaeaaaf8a2d;p=linux-2.6 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 Acked-by: Herbert Xu Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index d32c14de27..37a642c548 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -174,10 +174,13 @@ struct rcu_head { * 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