Impose a full barrier in generic-xlc.h atomics functions.

pg_atomic_compare_exchange_*_impl() were providing only the semantics of
an acquire barrier.  Buildfarm members hornet and mandrill revealed this
deficit beginning with commit 008608b9d5.
While we have no report of symptoms in 9.5, we can't rule out the
possibility of certain compilers, hardware, or extension code relying on
these functions' specified barrier semantics.  Back-patch to 9.5, where
commit b64d92f1a5 introduced atomics.

Reviewed by Andres Freund.

src/include/port/atomics/generic-xlc.h

@@ -46,11 +46,21 @@ static inline bool
 pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
 									uint32 *expected, uint32 newval)
 {
+	bool		ret;
+
+	/*
+	 * atomics.h specifies sequential consistency ("full barrier semantics")
+	 * for this interface.  Since "lwsync" provides acquire/release
+	 * consistency only, do not use it here.  GCC atomics observe the same
+	 * restriction; see its rs6000_pre_atomic_barrier().
+	 */
+	__asm__ __volatile__ ("	sync \n" ::: "memory");
+
 	/*
 	 * XXX: __compare_and_swap is defined to take signed parameters, but that
 	 * shouldn't matter since we don't perform any arithmetic operations.
 	 */
-	bool		ret = __compare_and_swap((volatile int*)&ptr->value,
+	ret = __compare_and_swap((volatile int*)&ptr->value,
 							 (int *)expected, (int)newval);
 
 	/*
@@ -69,6 +79,10 @@ pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
 static inline uint32
 pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
 {
+	/*
+	 * __fetch_and_add() emits a leading "sync" and trailing "isync", thereby
+	 * providing sequential consistency.  This is undocumented.
+	 */
 	return __fetch_and_add((volatile int *)&ptr->value, add_);
 }
 
@@ -79,7 +93,11 @@ static inline bool
 pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 									uint64 *expected, uint64 newval)
 {
-	bool		ret = __compare_and_swaplp((volatile long*)&ptr->value,
+	bool		ret;
+
+	__asm__ __volatile__ ("	sync \n" ::: "memory");
+
+	ret = __compare_and_swaplp((volatile long*)&ptr->value,
 							   (long *)expected, (long)newval);
 
 	__isync();