From 785be453f64c79ef054021a4c8b376d0b6a98fd1 Mon Sep 17 00:00:00 2001
From: Gary Pennington <garypen@gmail.com>
Date: Wed, 14 Jun 2023 15:23:12 +0100
Subject: [PATCH] remove a race from the hashing write()

Calling load() and store() separately leaves a lot of opportunity for a
race to occur.

This changes the logic to use compare_exchange() and will only return if
the previous value is absolutely not 1 at the point of comparison.

I also don't think this needs to be SeqCst, so relaxing that requirement
a little.
---
 src/config/hashing.rs | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/src/config/hashing.rs b/src/config/hashing.rs
index 2ac19199..986ed836 100644
--- a/src/config/hashing.rs
+++ b/src/config/hashing.rs
@@ -50,14 +50,17 @@ impl HokmaLock {
 
     pub fn write(&'static self) -> WhenTheHokmaSuppression {
         loop {
-            let previous = self.lock.load(Ordering::SeqCst);
-            self.lock.store(1, Ordering::SeqCst);
-
-            if previous != 1 {
-                return WhenTheHokmaSuppression {
-                    hokma: self,
-                    state: previous,
-                };
+            // We are only interested in error results
+            if let Err(previous) = self
+                .lock
+                .compare_exchange(1, 1, Ordering::Acquire, Ordering::Relaxed)
+            {
+                if previous != 1 {
+                    return WhenTheHokmaSuppression {
+                        hokma: self,
+                        state: previous,
+                    };
+                }
             }
         }
     }