From bf07964fea766120f88b8f86a38bcca84422169c Mon Sep 17 00:00:00 2001
From: Markus Pilman <markus.pilman@snowflake.com>
Date: Mon, 13 Jul 2020 11:38:00 -0600
Subject: [PATCH] Make ThreadSpinLock large to prevent false sharing

This fixes #3161
---
 flow/ThreadPrimitives.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/flow/ThreadPrimitives.h b/flow/ThreadPrimitives.h
index 6e43b8f1ab..35d3f4c287 100644
--- a/flow/ThreadPrimitives.h
+++ b/flow/ThreadPrimitives.h
@@ -23,6 +23,7 @@
 #pragma once
 
 #include <atomic>
+#include <array>
 
 #include "flow/Error.h"
 #include "flow/Trace.h"
@@ -44,6 +45,10 @@
 #include <drd.h>
 #endif
 
+// TODO: We should make this dependent on the CPU. Maybe cmake
+// can set this variable properly?
+constexpr size_t CACHE_LINE_SIZE = 64;
+
 class ThreadSpinLock {
 public:
 // #ifdef _WIN32
@@ -83,6 +88,9 @@ private:
 	ThreadSpinLock(const ThreadSpinLock&);
 	void operator=(const ThreadSpinLock&);
 	std::atomic_flag isLocked = ATOMIC_FLAG_INIT;
+	// We want a spin lock to occupy a cache line in order to
+	// prevent false sharing.
+	std::array<uint8_t, CACHE_LINE_SIZE - sizeof(isLocked)> padding;
 };
 
 class ThreadSpinLockHolder {