Require memory barrier support.

Previously we had a fallback implementation that made a harmless system
call, based on the assumption that system calls must contain a memory
barrier.  That shouldn't be reached on any current system, and it seems
highly likely that we can easily find out how to request explicit memory
barriers, if we've already had to find out how to do atomics on a
hypothetical new system.

Removed comments and a function name referred to a spinlock used for
fallback memory barriers, but that changed in 1b468a13, which left some
misleading words behind in a few places.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Suggested-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/721bf39a-ed8a-44b0-8b8e-be3bd81db748%40technowledgy.de
Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.us

3 files changed, 4 insertions, 26 deletions

diff --git a/src/include/port/atomics.h b/src/include/port/atomics.h
index edb0ae40dc0..c0c8688f736 100644
--- a/src/include/port/atomics.h
+++ b/src/include/port/atomics.h
@@ -101,6 +101,10 @@
 #if !defined(pg_compiler_barrier_impl)
 #error "could not find an implementation of pg_compiler_barrier"
 #endif
+#if !defined(pg_memory_barrier_impl)
+#error "could not find an implementation of pg_memory_barrier_impl"
+#endif
+
 
 /*
  * Provide a spinlock-based implementation of the 64 bit variants, if
diff --git a/src/include/port/atomics/fallback.h b/src/include/port/atomics/fallback.h
index 9f83827d83f..2c0eb287686 100644
--- a/src/include/port/atomics/fallback.h
+++ b/src/include/port/atomics/fallback.h
@@ -17,22 +17,6 @@
 #	error "should be included via atomics.h"
 #endif
 
-#ifndef pg_memory_barrier_impl
-/*
- * If we have no memory barrier implementation for this architecture, we
- * fall back to acquiring and releasing a spinlock.
- *
- * It's not self-evident that every possible legal implementation of a
- * spinlock acquire-and-release would be equivalent to a full memory barrier.
- * For example, I'm not sure that Itanium's acq and rel add up to a full
- * fence.  But all of our actual implementations seem OK in this regard.
- */
-#define PG_HAVE_MEMORY_BARRIER_EMULATION
-
-extern void pg_spinlock_barrier(void);
-#define pg_memory_barrier_impl pg_spinlock_barrier
-#endif
-
 
 #if !defined(PG_HAVE_ATOMIC_U64_SUPPORT)
 
diff --git a/src/include/port/atomics/generic.h b/src/include/port/atomics/generic.h
index 6113ab62a31..b636f951423 100644
--- a/src/include/port/atomics/generic.h
+++ b/src/include/port/atomics/generic.h
@@ -135,19 +135,9 @@ pg_atomic_unlocked_test_flag_impl(volatile pg_atomic_flag *ptr)
 static inline void
 pg_atomic_clear_flag_impl(volatile pg_atomic_flag *ptr)
 {
-	/*
-	 * Use a memory barrier + plain write if we have a native memory
-	 * barrier. But don't do so if memory barriers use spinlocks - that'd lead
-	 * to circularity if flags are used to implement spinlocks.
-	 */
-#ifndef PG_HAVE_MEMORY_BARRIER_EMULATION
 	/* XXX: release semantics suffice? */
 	pg_memory_barrier_impl();
 	pg_atomic_write_u32_impl(ptr, 0);
-#else
-	uint32 value = 1;
-	pg_atomic_compare_exchange_u32_impl(ptr, &value, 0);
-#endif
 }
 
 #elif !defined(PG_HAVE_ATOMIC_TEST_SET_FLAG)