Messages in this thread Patch in this message | | | Date | Fri, 30 Nov 2007 16:37:21 -0800 | From | "Paul E. McKenney" <> | Subject | [PATCH] Remove rcu_assign_pointer() penalty for NULL pointers |
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Hello!
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> ---
rcupdate.h | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)
diff -urpNa -X dontdiff linux-2.6.24-rc1-ego/include/linux/rcupdate.h linux-2.6.24-rc1-egoxu/include/linux/rcupdate.h --- linux-2.6.24-rc1-ego/include/linux/rcupdate.h 2007-11-06 15:30:02.000000000 -0800 +++ linux-2.6.24-rc1-egoxu/include/linux/rcupdate.h 2007-11-30 09:06:11.000000000 -0800 @@ -191,10 +191,13 @@ static inline void rcu_preempt_boost(voi * 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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