From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Mon, 19 Apr 2021 15:21:52 +0000 (-0400)
Subject: Fix: use __atomic_load() rather than atomic load explicit
X-Git-Tag: v0.13.0~13
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=3abcdbd14d1b9291f06a4a4bbca313fcfbe5452b;p=userspace-rcu.git

Fix: use __atomic_load() rather than atomic load explicit

Use __atomic_load (gcc extension) rather than atomic load explict
(C11/C++11) for rcu_dereference because it does not require the input
type to be _Atomic. This fixes a regression with clang introduced by
commit 380f4b19052 ("Fix: use atomic load memory_order_consume for
rcu_dereference on C11/C++11").

Note that the cmm_smp_read_barrier_depends is removed when using
__ATOMIC_CONSUME because their memory ordering effect is redundant.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Icd5f6040e0bd2167844a8d856ae5475ceef17123
---

diff --git a/include/urcu/static/pointer.h b/include/urcu/static/pointer.h
index 891daed..ca9a2f9 100644
--- a/include/urcu/static/pointer.h
+++ b/include/urcu/static/pointer.h
@@ -61,6 +61,10 @@ extern "C" {
  * optimizations are taken care of by the "memory_order_consume" atomic
  * load.
  *
+ * Use the gcc __atomic_load() rather than C11/C++11 atomic load
+ * 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.
@@ -80,33 +84,25 @@ extern "C" {
  * expanded directly in non-LGPL code.
  */
 
-#ifdef URCU_DEREFERENCE_USE_VOLATILE
-# define __rcu_dereference(p)	CMM_LOAD_SHARED(p)
-#else
-# if defined (__cplusplus)
-#  if __cplusplus >= 201103L
-#   include <atomic>
-#   define __rcu_dereference(p)	((std::atomic<__typeof__(p)>)(p)).load(std::memory_order_consume)
-#  else
-#   define __rcu_dereference(p)	CMM_LOAD_SHARED(x)
-#  endif
-# else
-#  if (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L)
-#   include <stdatomic.h>
-#   define __rcu_dereference(p)	atomic_load_explicit(&(p), memory_order_consume)
-#  else
-#   define __rcu_dereference(p)	CMM_LOAD_SHARED(p)
-#  endif
-# endif
+#if !defined (URCU_DEREFERENCE_USE_VOLATILE) &&		\
+	((defined (__cplusplus) && __cplusplus >= 201103L) ||	\
+	(defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L))
+# define __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
 #endif
 
-#define _rcu_dereference(p)						\
-				__extension__				\
-				({					\
-				__typeof__(p) _________p1 = __rcu_dereference(p); \
-				cmm_smp_read_barrier_depends();		\
-				(_________p1);				\
-				})
+#ifdef __URCU_DEREFERENCE_USE_ATOMIC_CONSUME
+# define _rcu_dereference(p) __extension__ ({						\
+				__typeof__(p) _________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
 
 /**
  * _rcu_cmpxchg_pointer - same as rcu_assign_pointer, but tests if the pointer