Commit d99c4f6b authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Linus Torvalds

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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent ba6f867f
...@@ -174,10 +174,13 @@ struct rcu_head { ...@@ -174,10 +174,13 @@ struct rcu_head {
* code. * code.
*/ */
#define rcu_assign_pointer(p, v) ({ \ #define rcu_assign_pointer(p, v) \
smp_wmb(); \ ({ \
(p) = (v); \ if (!__builtin_constant_p(v) || \
}) ((v) != NULL)) \
smp_wmb(); \
(p) = (v); \
})
/** /**
* synchronize_sched - block until all CPUs have exited any non-preemptive * synchronize_sched - block until all CPUs have exited any non-preemptive
......
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