From 19353d3e68be31ea8d6627696c2d95243725082c Mon Sep 17 00:00:00 2001
From: Chirantan Ekbote <chirantan@chromium.org>
Date: Mon, 5 Apr 2021 18:55:40 +0900
Subject: [PATCH] sync: spinlock: Check state before compare_exchange_weak

Checking the state with a relaxed load before doing a
compare_exchange_weak can reduce unnecessary coherence traffic on the
CPU and improve performance.

BUG=none
TEST=unit tests

Change-Id: Icabd9863ceb5ba674dbec601afee8f7962f69413
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2805753
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Commit-Queue: Chirantan Ekbote <chirantan@chromium.org>
---
 cros_async/src/sync/spin.rs | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/cros_async/src/sync/spin.rs b/cros_async/src/sync/spin.rs
index 4fb6172699..8b7b811939 100644
--- a/cros_async/src/sync/spin.rs
+++ b/cros_async/src/sync/spin.rs
@@ -54,11 +54,16 @@ impl<T: ?Sized> SpinLock<T> {
     /// `SpinLockGuard` is dropped. Attempting to call `lock` while already holding the `SpinLock`
     /// will cause a deadlock.
     pub fn lock(&self) -> SpinLockGuard<T> {
-        while self
-            .lock
-            .compare_exchange_weak(UNLOCKED, LOCKED, Ordering::Acquire, Ordering::Relaxed)
-            .is_err()
-        {
+        loop {
+            let state = self.lock.load(Ordering::Relaxed);
+            if state == UNLOCKED
+                && self
+                    .lock
+                    .compare_exchange_weak(UNLOCKED, LOCKED, Ordering::Acquire, Ordering::Relaxed)
+                    .is_ok()
+            {
+                break;
+            }
             spin_loop_hint();
         }