Gitweb:
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=d99c4f6b13b3149bc83703ab1493beaeaaaf8a2d
Commit: d99c4f6b13b3149bc83703ab1493beaeaaaf8a2d
Parent: ba6f867f114760d4e43f0f93abe280ee0a0d696e
Author: Paul E. McKenney <[EMAIL PROTECTED]>
AuthorDate: Wed Feb 6 01:37:25 2008 -0800
Committer: Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Wed Feb 6 10:41:06 2008 -0800
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/[EMAIL PROTECTED]/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 <[EMAIL PROTECTED]>
Acked-by: Herbert Xu <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
---
include/linux/rcupdate.h | 11 +++++++----
1 files changed, 7 insertions(+), 4 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index d32c14d..37a642c 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
-
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