v19-0001-Read-only-atomic-backend-write-function.patch

From b1cd3358991553b50a087e6c0b865d8733d82951 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Mon, 11 Oct 2021 16:15:06 -0400
Subject: [PATCH v19 1/9] 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 856338f161..af30da32e5 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 */
-- 
2.30.2