From 53f717e84a3aa912f9d89dce4be7f77d9bffb3c2 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Fri, 24 Nov 2023 20:21:12 -0600
Subject: [PATCH v2 1/2] add membarrier operations

---
 src/include/port/atomics.h         | 59 ++++++++++++++++++++++++++++++
 src/include/port/atomics/generic.h | 36 ++++++++++++++++++
 2 files changed, 95 insertions(+)

diff --git a/src/include/port/atomics.h b/src/include/port/atomics.h
index bbff945eba..a889f0236e 100644
--- a/src/include/port/atomics.h
+++ b/src/include/port/atomics.h
@@ -239,6 +239,26 @@ pg_atomic_read_u32(volatile pg_atomic_uint32 *ptr)
 	return pg_atomic_read_u32_impl(ptr);
 }
 
+/*
+ * pg_atomic_read_membarrier_u32 - read with barrier semantics.
+ *
+ * This read is guaranteed to return the current value, provided that the value
+ * is only ever updated via operations with barrier semantics, such as
+ * pg_atomic_compare_exchange_u32() and pg_atomic_write_membarrier_u32().  Note
+ * that this is less performant than pg_atomic_read_u32(), but it may be easier
+ * to reason about correctness with this function in less performance-sensitive
+ * code.
+ *
+ * Full barrier semantics.
+ */
+static inline uint32
+pg_atomic_read_membarrier_u32(volatile pg_atomic_uint32 *ptr)
+{
+	AssertPointerAlignment(ptr, 4);
+
+	return pg_atomic_read_membarrier_u32_impl(ptr);
+}
+
 /*
  * pg_atomic_write_u32 - write to atomic variable.
  *
@@ -276,6 +296,27 @@ pg_atomic_unlocked_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
 	pg_atomic_unlocked_write_u32_impl(ptr, val);
 }
 
+/*
+ * pg_atomic_write_membarrier_u32 - write with barrier semantics.
+ *
+ * The write is guaranteed to succeed as a whole, i.e., it's not possible to
+ * observe a partial write for any reader.  Note that this correctly interacts
+ * with both pg_atomic_compare_exchange_u32() and
+ * pg_atomic_read_membarrier_u32().  While this may be less performant than
+ * pg_atomic_write_u32() and pg_atomic_unlocked_write_u32(), it may be easier
+ * to reason about correctness with this function in less performance-sensitive
+ * code.
+ *
+ * Full barrier semantics.
+ */
+static inline void
+pg_atomic_write_membarrier_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
+{
+	AssertPointerAlignment(ptr, 4);
+
+	pg_atomic_write_membarrier_u32_impl(ptr, val);
+}
+
 /*
  * pg_atomic_exchange_u32 - exchange newval with current value
  *
@@ -429,6 +470,15 @@ pg_atomic_read_u64(volatile pg_atomic_uint64 *ptr)
 	return pg_atomic_read_u64_impl(ptr);
 }
 
+static inline uint64
+pg_atomic_read_membarrier_u64(volatile pg_atomic_uint64 *ptr)
+{
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
+	AssertPointerAlignment(ptr, 8);
+#endif
+	return pg_atomic_read_membarrier_u64_impl(ptr);
+}
+
 static inline void
 pg_atomic_write_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
 {
@@ -438,6 +488,15 @@ pg_atomic_write_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
 	pg_atomic_write_u64_impl(ptr, val);
 }
 
+static inline void
+pg_atomic_write_membarrier_u64(volatile pg_atomic_uint64 *ptr, uint64 val)
+{
+#ifndef PG_HAVE_ATOMIC_U64_SIMULATION
+	AssertPointerAlignment(ptr, 8);
+#endif
+	pg_atomic_write_membarrier_u64_impl(ptr, val);
+}
+
 static inline uint64
 pg_atomic_exchange_u64(volatile pg_atomic_uint64 *ptr, uint64 newval)
 {
diff --git a/src/include/port/atomics/generic.h b/src/include/port/atomics/generic.h
index 95d99dd0be..9bcc3b48ce 100644
--- a/src/include/port/atomics/generic.h
+++ b/src/include/port/atomics/generic.h
@@ -243,6 +243,24 @@ pg_atomic_sub_fetch_u32_impl(volatile pg_atomic_uint32 *ptr, int32 sub_)
 }
 #endif
 
+#if !defined(PG_HAVE_ATOMIC_READ_MEMBARRIER_U32) && defined(PG_HAVE_ATOMIC_FETCH_ADD_U32)
+#define PG_HAVE_ATOMIC_READ_MEMBARRIER_U32
+static inline uint32
+pg_atomic_read_membarrier_u32_impl(volatile pg_atomic_uint32 *ptr)
+{
+	return pg_atomic_fetch_add_u32_impl(ptr, 0);
+}
+#endif
+
+#if !defined(PG_HAVE_ATOMIC_WRITE_MEMBARRIER_U32) && defined(PG_HAVE_ATOMIC_EXCHANGE_U32)
+#define PG_HAVE_ATOMIC_WRITE_MEMBARRIER_U32
+static inline void
+pg_atomic_write_membarrier_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val)
+{
+	(void) pg_atomic_exchange_u32_impl(ptr, val);
+}
+#endif
+
 #if !defined(PG_HAVE_ATOMIC_EXCHANGE_U64) && defined(PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U64)
 #define PG_HAVE_ATOMIC_EXCHANGE_U64
 static inline uint64
@@ -399,3 +417,21 @@ pg_atomic_sub_fetch_u64_impl(volatile pg_atomic_uint64 *ptr, int64 sub_)
 	return pg_atomic_fetch_sub_u64_impl(ptr, sub_) - sub_;
 }
 #endif
+
+#if !defined(PG_HAVE_ATOMIC_READ_MEMBARRIER_U64) && defined(PG_HAVE_ATOMIC_FETCH_ADD_U64)
+#define PG_HAVE_ATOMIC_READ_MEMBARRIER_U64
+static inline uint64
+pg_atomic_read_membarrier_u64_impl(volatile pg_atomic_uint64 *ptr)
+{
+	return pg_atomic_fetch_add_u64_impl(ptr, 0);
+}
+#endif
+
+#if !defined(PG_HAVE_ATOMIC_WRITE_MEMBARRIER_U64) && defined(PG_HAVE_ATOMIC_EXCHANGE_U64)
+#define PG_HAVE_ATOMIC_WRITE_MEMBARRIER_U64
+static inline void
+pg_atomic_write_membarrier_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 val)
+{
+	(void) pg_atomic_exchange_u64_impl(ptr, val);
+}
+#endif
-- 
2.25.1