The patch titled
Remove rcu_assign_pointer() penalty for NULL pointers
has been added to the -mm tree. Its filename is
remove-rcu_assign_pointer-penalty-for-null-pointers.patch
*** Remember to use Documentation/SubmitChecklist when testing your code ***
See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this
------------------------------------------------------
Subject: Remove rcu_assign_pointer() penalty for NULL pointers
From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxx/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@xxxxxxxxxxxxxxxxxx>
Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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
@@ -272,10 +272,13 @@ extern struct lockdep_map rcu_lock_map;
* 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
_
Patches currently in -mm which might be from paulmck@xxxxxxxxxxxxxxxxxx are
remove-rcu_assign_pointer-penalty-for-null-pointers.patch
-
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