4 files changed, 48 insertions, 2 deletions
diff --git a/src/backend/port/atomics.c b/src/backend/port/atomics.c
index 439b3c18c7b..aa15df5a47a 100644
--- a/src/backend/port/atomics.c
+++ b/src/backend/port/atomics.c
@@ -112,6 +112,19 @@ pg_atomic_init_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val_)
 	ptr->value = val_;
 }
 
+void
+pg_atomic_write_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val)
+{
+	/*
+	 * One might think that an unlocked write doesn't need to acquire the
+	 * spinlock, but one would be wrong. Even an unlocked write has to cause a
+	 * concurrent pg_atomic_compare_exchange_u32() (et al) to fail.
+	 */
+	SpinLockAcquire((slock_t *) &ptr->sema);
+	ptr->value = val;
+	SpinLockRelease((slock_t *) &ptr->sema);
+}
+
 bool
 pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
 									uint32 *expected, uint32 newval)
diff --git a/src/include/port/atomics.h b/src/include/port/atomics.h
index d94fea60f15..0b506fcbc0b 100644
--- a/src/include/port/atomics.h
+++ b/src/include/port/atomics.h
@@ -297,10 +297,12 @@ pg_atomic_read_u32(volatile pg_atomic_uint32 *ptr)
 }
 
 /*
- * pg_atomic_write_u32 - unlocked write to atomic variable.
+ * pg_atomic_write_u32 - write to atomic variable.
  *
  * The write is guaranteed to succeed as a whole, i.e. it's not possible to
- * observe a partial write for any reader.
+ * observe a partial write for any reader.  Note that this correctly interacts
+ * with pg_atomic_compare_exchange_u32, in contrast to
+ * pg_atomic_unlocked_write_u32().
  *
  * No barrier semantics.
  */
@@ -313,6 +315,25 @@ pg_atomic_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
 }
 
 /*
+ * pg_atomic_unlocked_write_u32 - unlocked write to atomic variable.
+ *
+ * The write is guaranteed to succeed as a whole, i.e. it's not possible to
+ * observe a partial write for any reader.  But note that writing this way is
+ * not guaranteed to correctly interact with read-modify-write operations like
+ * pg_atomic_compare_exchange_u32.  This should only be used in cases where
+ * minor performance regressions due to atomics emulation are unacceptable.
+ *
+ * No barrier semantics.
+ */
+static inline void
+pg_atomic_unlocked_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
+{
+	AssertPointerAlignment(ptr, 4);
+
+	pg_atomic_unlocked_write_u32_impl(ptr, val);
+}
+
+/*
  * pg_atomic_exchange_u32 - exchange newval with current value
  *
  * Returns the old value of 'ptr' before the swap.
diff --git a/src/include/port/atomics/fallback.h b/src/include/port/atomics/fallback.h
index 4e04f9758b9..a1380034ba9 100644
--- a/src/include/port/atomics/fallback.h
+++ b/src/include/port/atomics/fallback.h
@@ -135,6 +135,9 @@ pg_atomic_unlocked_test_flag_impl(volatile pg_atomic_flag *ptr)
 #define PG_HAVE_ATOMIC_INIT_U32
 extern void pg_atomic_init_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val_);
 
+#define PG_HAVE_ATOMIC_WRITE_U32
+extern void pg_atomic_write_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val);
+
 #define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U32
 extern bool pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
 												uint32 *expected, uint32 newval);
diff --git a/src/include/port/atomics/generic.h b/src/include/port/atomics/generic.h
index bb31df36237..ec140e4a1e0 100644
--- a/src/include/port/atomics/generic.h
+++ b/src/include/port/atomics/generic.h
@@ -60,6 +60,15 @@ pg_atomic_write_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val)
 }
 #endif
 
+#ifndef PG_HAVE_ATOMIC_UNLOCKED_WRITE_U32
+#define PG_HAVE_ATOMIC_UNLOCKED_WRITE_U32
+static inline void
+pg_atomic_unlocked_write_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 val)
+{
+	ptr->value = val;
+}
+#endif
+
 /*
  * provide fallback for test_and_set using atomic_exchange if available
  */