From: "Paul E. McKenney" 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 --- include/linux/rcupdate.h | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff -puN include/linux/rcupdate.h~remove-rcu_assign_pointer-penalty-for-null-pointers include/linux/rcupdate.h --- a/include/linux/rcupdate.h~remove-rcu_assign_pointer-penalty-for-null-pointers +++ a/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 _