From 37e385e7832c3dd897a583e648300b5a1d5d2cd1 Mon Sep 17 00:00:00 2001
From: "Justin C. Miller" <justin@devjustinian.com>
Date: Sun, 11 Jul 2021 18:11:47 -0700
Subject: [PATCH] [kutil] Fix spinlock release during contention

Spinlock release uses __atomic_compare_exchange_n, which overwrites the
`desired` parameter with the actual value when the compare fails. This
was causing releases to always spin when there was lock contention.
---
 src/libraries/kutil/include/kutil/spinlock.h |  2 +-
 src/libraries/kutil/spinlock.cpp             | 21 +++++++++-----------
 2 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/src/libraries/kutil/include/kutil/spinlock.h b/src/libraries/kutil/include/kutil/spinlock.h
index 68a059c..96bbfbe 100644
--- a/src/libraries/kutil/include/kutil/spinlock.h
+++ b/src/libraries/kutil/include/kutil/spinlock.h
@@ -15,7 +15,7 @@ public:
 	/// A node in the wait queue.
 	struct waiter
 	{
-		bool locked;
+		bool blocked;
 		waiter *next;
 	};
 
diff --git a/src/libraries/kutil/spinlock.cpp b/src/libraries/kutil/spinlock.cpp
index 6c549b5..73b57de 100644
--- a/src/libraries/kutil/spinlock.cpp
+++ b/src/libraries/kutil/spinlock.cpp
@@ -11,7 +11,7 @@ void
 spinlock::acquire(waiter *w)
 {
 	w->next = nullptr;
-	w->locked = true;
+	w->blocked = true;
 
 	// Point the lock at this waiter
 	waiter *prev = __atomic_exchange_n(&m_lock, w, memorder);
@@ -19,30 +19,27 @@ spinlock::acquire(waiter *w)
 		// If there was a previous waiter, wait for them to
 		// unblock us
 		prev->next = w;
-		while (w->locked) {
+		while (w->blocked)
 			asm ("pause");
-		}
 	} else {
-		w->locked = false;
+		w->blocked = false;
 	}
 }
 
 void
 spinlock::release(waiter *w)
 {
-	if (!w->next) {
-		// If we're still the last waiter, we're done
-		if(__atomic_compare_exchange_n(&m_lock, &w, nullptr, false, memorder, memorder))
-			return;
-	}
+	// If we're still the last waiter, we're done
+	waiter *expected = w;
+	if(__atomic_compare_exchange_n(&m_lock, &expected, nullptr, false, memorder, memorder))
+		return;
 
 	// Wait for the subseqent waiter to tell us who they are
-	while (!w->next) {
+	while (!w->next)
 		asm ("pause");
-	}
 
 	// Unblock the subseqent waiter
-	w->next->locked = false;
+	w->next->blocked = false;
 }