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Remove rcu_assign_pointer() penalty for NULL pointers

PATCH In review Paul E. McKenney 2 participants 3 messages Dec 1, 2007

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Networking Network drivers Ethernet bridge RCU
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Paul E. McKenney

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#0 lore raw

PATCH 1/1 Remove rcu_assign_pointer() penalty for NULL pointers

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/[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]>

---

 rcupdate.h |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)
include/linux/rcupdate.h +7 -4
diff -urpNa -X dontdiff linux-2.6.24-rc1-ego/include/linux/rcupdate.h linux-2.6.24-rc1-egoxu/include/linux/rcupdate.h
@@ -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
Acked-by on v1 1/1
#1 lore raw
Quoted 9 lines · Paul E. McKenney

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:

Quoted 7 lines
	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]>

Thanks a lot for following through with this Paul!

Signature · 4 lines
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Email: Herbert Xu ~{PmV>HI~} <[email protected]>
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#2 lore raw
Quoted 16 lines · Herbert Xu
Quoted 15 lines · Paul E. McKenney

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.

Quoted 5 lines
Quoted 1 line
Signed-off-by Paul E. McKenney <[email protected]>
Acked-by Herbert Xu <[email protected]>

Thanks a lot for following through with this Paul!

No problem -- after all, it is not every day that one gets the opportunity to make a simple change that speeds things up and makes kernel hackers lives a bit simpler. ;-)

Thanx, Paul