From bc4afef0bf0cb34d90fb6c029ab4c5ff1a6d033d Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Mon, 11 Oct 2021 16:15:06 -0400
Subject: [PATCH 1/8] Read-only atomic backend write function

For counters in shared memory which can be read by any backend but only
written to by one backend, an atomic is still needed to protect against
torn values; however, pg_atomic_fetch_add_u64() is overkill for
incrementing such counters. pg_atomic_unlocked_inc_counter() is a helper
function which can be used to increment these values safely without
unnecessary overhead.

Author: Thomas Munro <tmunro@postgresql.org>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://www.postgresql.org/message-id/CA%2BhUKGJ06d3h5JeOtAv4h52n0vG1jOPZxqMCn5FySJQUVZA32w%40mail.gmail.com
Discussion: https://www.postgresql.org/message-id/flat/20200124195226.lth52iydq2n2uilq%40alap3.anarazel.de
---
 src/include/port/atomics.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/include/port/atomics.h b/src/include/port/atomics.h
index 9550e04aaa5..3d1fdc64752 100644
--- a/src/include/port/atomics.h
+++ b/src/include/port/atomics.h
@@ -519,6 +519,17 @@ pg_atomic_sub_fetch_u64(volatile pg_atomic_uint64 *ptr, int64 sub_)
 	return pg_atomic_sub_fetch_u64_impl(ptr, sub_);
 }
 
+/*
+ * On modern systems this is really just *counter++. On some older systems
+ * there might be more to it (due to an inability to read and write 64-bit
+ * values atomically).
+ */
+static inline void
+pg_atomic_unlocked_inc_counter(pg_atomic_uint64 *counter)
+{
+	pg_atomic_write_u64(counter, pg_atomic_read_u64(counter) + 1);
+}
+
 #undef INSIDE_ATOMICS_H
 
 #endif							/* ATOMICS_H */
-- 
2.17.1

