Remove rcu_assign_pointer() penalty for NULL pointers
commitd99c4f6b13b3149bc83703ab1493beaeaaaf8a2d
authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Wed, 6 Feb 2008 09:37:25 +0000 (6 01:37 -0800)
committerLinus Torvalds <torvalds@woody.linux-foundation.org>
Wed, 6 Feb 2008 18:41:06 +0000 (6 10:41 -0800)
tree32e09d76cb46755d7420e6ad9a6e0802dab47963
parentba6f867f114760d4e43f0f93abe280ee0a0d696e
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>
include/linux/rcupdate.h