Re: [PATCH 2/2] target: use RCU_INIT_POINTER() when NULLing.

[Paul E. McKenney]

On Sun, May 01, 2016 at 10:01:20AM -0700, Christoph Hellwig wrote:
> On Sun, May 01, 2016 at 06:22:01PM +0530, Muhammad Falak R Wani wrote:
> > It is safe to use RCU_INIT_POINTER() to NULL, instead of
> > rcu_assign_pointer().
> > This results in slightly smaller/faster code.
> 
> If this is indeed the case, rcu_assign_pointer should simply check
> for NULL using __builtin_constant_p and do the right thing transparently
> instead of burdening it on every user.

Last time around, there was a compiler bug that prevented this from
working correctly.  But it could well be time to look at it again.
How does the following (untested) patch look?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index c61b6b9506e7..3a4dbfe63c1a 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -650,7 +650,16 @@ static inline void rcu_preempt_sleep_check(void)
  * please be careful when making changes to rcu_assign_pointer() and the
  * other macros that it invokes.
  */
-#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
+#define rcu_assign_pointer(p, v) \
+({ \
+	typeof(v) _r_a_p__v = (v); \
+	\
+	if (__builtin_constant_p(v) && (_r_a_p__v) == NULL) \
+		WRITE_ONCE((p), (_r_a_p__v)); \
+	else \
+		smp_store_release(&p, RCU_INITIALIZER(_r_a_p__v)); \
+	_r_a_p__v; \
+})
 
 /**
  * rcu_access_pointer() - fetch RCU pointer with no dereferencing