[lttng-dev] [PATCH 4/7] Replace the internal pointer manipulation with __atomic builtins

[Ondřej Surý]

Instead of custom code, use the __atomic builtins to implement the
rcu_dereference(), rcu_cmpxchg_pointer(), rcu_xchg_pointer() and
rcu_assign_pointer().

The rcu_dereference() now relies on CONSUME memory order.

Signed-off-by: Ondřej Surý <ondrej at sury.org>
---
 include/urcu/static/pointer.h | 77 +++++++----------------------------
 1 file changed, 14 insertions(+), 63 deletions(-)

diff --git a/include/urcu/static/pointer.h b/include/urcu/static/pointer.h
index 9e46a57..a549483 100644
--- a/include/urcu/static/pointer.h
+++ b/include/urcu/static/pointer.h
@@ -38,6 +38,8 @@
 extern "C" {
 #endif
 
+#define _rcu_get_pointer(addr) __atomic_load_n(addr, __ATOMIC_CONSUME)
+
 /**
  * _rcu_dereference - reads (copy) a RCU-protected pointer to a local variable
  * into a RCU read-side critical section. The pointer can later be safely
@@ -49,14 +51,6 @@ extern "C" {
  * Inserts memory barriers on architectures that require them (currently only
  * Alpha) and documents which pointers are protected by RCU.
  *
- * With C standards prior to C11/C++11, the compiler memory barrier in
- * CMM_LOAD_SHARED() ensures that value-speculative optimizations (e.g.
- * VSS: Value Speculation Scheduling) does not perform the data read
- * before the pointer read by speculating the value of the pointer.
- * Correct ordering is ensured because the pointer is read as a volatile
- * access. This acts as a global side-effect operation, which forbids
- * reordering of dependent memory operations.
- *
  * With C standards C11/C++11, concerns about dependency-breaking
  * optimizations are taken care of by the "memory_order_consume" atomic
  * load.
@@ -65,10 +59,6 @@ extern "C" {
  * explicit because the pointer used as input argument is a pointer,
  * not an _Atomic type as required by C11/C++11.
  *
- * By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of
- * volatile access to implement rcu_dereference rather than
- * memory_order_consume load from the C11/C++11 standards.
- *
  * This may improve performance on weakly-ordered architectures where
  * the compiler implements memory_order_consume as a
  * memory_order_acquire, which is stricter than required by the
@@ -83,35 +73,7 @@ extern "C" {
  * meets the 10-line criterion in LGPL, allowing this function to be
  * expanded directly in non-LGPL code.
  */
-
-#if !defined (URCU_DEREFERENCE_USE_VOLATILE) &&		\
-	((defined (__cplusplus) && __cplusplus >= 201103L) ||	\
-	(defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L))
-# define __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
-#endif
-
-/*
- * If p is const (the pointer itself, not what it points to), using
- * __typeof__(p) would declare a const variable, leading to
- * -Wincompatible-pointer-types errors.  Using the statement expression
- * makes it an rvalue and gets rid of the const-ness.
- */
-#ifdef __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
-# define _rcu_dereference(p) __extension__ ({						\
-				__typeof__(__extension__ ({				\
-					__typeof__(p) __attribute__((unused)) _________p0 = { 0 }; \
-					_________p0;					\
-				})) _________p1;					\
-				__atomic_load(&(p), &_________p1, __ATOMIC_CONSUME);	\
-				(_________p1);						\
-			})
-#else
-# define _rcu_dereference(p) __extension__ ({						\
-				__typeof__(p) _________p1 = CMM_LOAD_SHARED(p);		\
-				cmm_smp_read_barrier_depends();				\
-				(_________p1);						\
-			})
-#endif
+#define _rcu_dereference(p) _rcu_get_pointer(&(p))
 
 /**
  * _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer
@@ -126,12 +88,12 @@ extern "C" {
  * meets the 10-line criterion in LGPL, allowing this function to be
  * expanded directly in non-LGPL code.
  */
-#define _rcu_cmpxchg_pointer(p, old, _new)				\
-	__extension__							\
-	({								\
-		__typeof__(*p) _________pold = (old);			\
-		__typeof__(*p) _________pnew = (_new);			\
-		uatomic_cmpxchg(p, _________pold, _________pnew);	\
+#define _rcu_cmpxchg_pointer(p, old, _new)						\
+	({										\
+		__typeof__(*(p)) __old = old;						\
+		__atomic_compare_exchange_n(p, &__old, _new, 0,				\
+					    __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);	\
+		__old;									\
 	})
 
 /**
@@ -145,22 +107,11 @@ extern "C" {
  * meets the 10-line criterion in LGPL, allowing this function to be
  * expanded directly in non-LGPL code.
  */
-#define _rcu_xchg_pointer(p, v)				\
-	__extension__					\
-	({						\
-		__typeof__(*p) _________pv = (v);	\
-		uatomic_xchg(p, _________pv);		\
-	})
-
+#define _rcu_xchg_pointer(p, v) \
+	__atomic_exchange_n(p, v, __ATOMIC_SEQ_CST)
 
-#define _rcu_set_pointer(p, v)				\
-	do {						\
-		__typeof__(*p) _________pv = (v);	\
-		if (!__builtin_constant_p(v) || 	\
-		    ((v) != NULL))			\
-			cmm_wmb();				\
-		uatomic_set(p, _________pv);		\
-	} while (0)
+#define _rcu_set_pointer(p, v) \
+	__atomic_store_n(p, v, __ATOMIC_RELEASE)
 
 /**
  * _rcu_assign_pointer - assign (publicize) a pointer to a new data structure
@@ -178,7 +129,7 @@ extern "C" {
  * meets the 10-line criterion in LGPL, allowing this function to be
  * expanded directly in non-LGPL code.
  */
-#define _rcu_assign_pointer(p, v)	_rcu_set_pointer(&(p), v)
+#define _rcu_assign_pointer(p, v) rcu_set_pointer(&(p), v)
 
 #ifdef __cplusplus
 }
-- 
2.39.2