v18-0001-Read-only-atomic-backend-write-function.patch

From 0a35d1bf2d70131b298adf0aa35349f13d8ad8aa Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Mon, 11 Oct 2021 16:15:06 -0400
Subject: [PATCH v18 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 the counter. pg_atomic_inc_counter() is a helper function
which can be used to increment these values safely but 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..dac9767b1c 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 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