Re: [lttng-dev] userspace-rcu and ThreadSanitizer

[Duncan Sands via lttng-dev <lttng-dev@lists.lttng.org>]

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Hi Ondřej,

> I have two questions:
> 
> 1. I've tried to help TSAN by replacing the custom atomics with __atomic gcc
>    primitives - that seems to work pretty well.  Note that using C11 stdatomics
>    is frankly not possible here because it would require wrapping everything into
>    _Atomic().

I'm using the attached patch which does much the same.

Ciao, Duncan.

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diff --git a/External/UserspaceRCU/userspace-rcu/include/urcu/system.h b/External/UserspaceRCU/userspace-rcu/include/urcu/system.h
index faae390554..82898d1dbe 100644
--- a/External/UserspaceRCU/userspace-rcu/include/urcu/system.h
+++ b/External/UserspaceRCU/userspace-rcu/include/urcu/system.h
@@ -26,34 +26,45 @@
  * Identify a shared load. A cmm_smp_rmc() or cmm_smp_mc() should come
  * before the load.
  */
-#define _CMM_LOAD_SHARED(p)	       CMM_ACCESS_ONCE(p)
+#define _CMM_LOAD_SHARED(p)					\
+	__extension__						\
+	({							\
+		__typeof__(p) v;				\
+		__atomic_load(&p, &v, __ATOMIC_RELAXED);	\
+		v;						\
+	})
 
 /*
  * Load a data from shared memory, doing a cache flush if required.
  */
-#define CMM_LOAD_SHARED(p)			\
-	__extension__			\
-	({				\
-		cmm_smp_rmc();		\
-		_CMM_LOAD_SHARED(p);	\
+#define CMM_LOAD_SHARED(p)					\
+	__extension__						\
+	({							\
+		__typeof__(p) v;				\
+		__atomic_load(&p, &v, __ATOMIC_ACQUIRE);	\
+		v;						\
 	})
 
 /*
  * Identify a shared store. A cmm_smp_wmc() or cmm_smp_mc() should
  * follow the store.
  */
-#define _CMM_STORE_SHARED(x, v)	__extension__ ({ CMM_ACCESS_ONCE(x) = (v); })
+#define _CMM_STORE_SHARED(x, v)					\
+	__extension__						\
+	({							\
+		__typeof__(x) w = v;				\
+		__atomic_store(&x, &w, __ATOMIC_RELAXED);	\
+	})
 
 /*
  * Store v into x, where x is located in shared memory. Performs the
  * required cache flush after writing. Returns v.
  */
-#define CMM_STORE_SHARED(x, v)						\
-	__extension__							\
-	({								\
-		__typeof__(x) _v = _CMM_STORE_SHARED(x, v);		\
-		cmm_smp_wmc();						\
-		_v = _v;	/* Work around clang "unused result" */	\
+#define CMM_STORE_SHARED(x, v)					\
+	__extension__						\
+	({							\
+		__typeof__(x) w = v;				\
+		__atomic_store(&x, &w, __ATOMIC_RELEASE);	\
 	})
 
 #endif /* _URCU_SYSTEM_H */

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