[kutil] Update spinlock to an MCS-style lock

Update the existing but unused spinlock class to an MCS-style queue
spinlock. This is probably still a WIP but I expect it to see more use
with SMP getting further integrated.

modules.yaml

@@ -114,6 +114,7 @@ modules:
             - src/libraries/kutil/logger.cpp
             - src/libraries/kutil/memory.cpp
             - src/libraries/kutil/printf.c
+            - src/libraries/kutil/spinlock.cpp
 
     cpu:
         kind: lib

src/libraries/kutil/include/kutil/spinlock.h

@@ -1,19 +1,20 @@
+/// \file spinlock.h
+/// Spinlock types and related defintions
+
 #pragma once
-#include <atomic>
 
 namespace kutil {
+namespace spinlock {
 
-
+/// An MCS based spinlock node
-class spinlock
+struct node
 {
-public:
+	bool locked;
-	spinlock() : m_lock(false) {}
+	node *next;
-
-	inline void enter() { while (!m_lock.exchange(true)); }
-	inline void leave() { m_lock.store(false); }
-
-private:
-	std::atomic<bool> m_lock;
 };
 
+void aquire(node *lock, node *waiter);
+void release(node *lock, node *waiter);
+
+} // namespace spinlock
 } // namespace kutil

src/libraries/kutil/spinlock.cpp

@@ -0,0 +1,48 @@
+#include "kutil/spinlock.h"
+
+namespace kutil {
+namespace spinlock {
+
+static constexpr int memorder = __ATOMIC_SEQ_CST;
+
+void
+aquire(node * &lock, node *waiter)
+{
+	waiter->next = nullptr;
+	waiter->locked = true;
+
+	// Point the lock at this waiter
+	node *prev = __atomic_exchange_n(&lock, waiter, memorder);
+	if (prev) {
+		// If there was a previous waiter, wait for them to
+		// unblock us
+		prev->next = waiter;
+		while (waiter->locked) {
+			asm ("pause");
+		}
+	} else {
+		waiter->locked = false;
+	}
+}
+
+void
+release(node * &lock, node *waiter)
+{
+	if (!waiter->next) {
+		// If we're still the last waiter, we're done
+		if(__atomic_compare_exchange_n(&lock, &waiter, nullptr, false, memorder, memorder))
+			return;
+	}
+
+	// Wait for the subseqent waiter to tell us who they are
+	while (!waiter->next) {
+		asm ("pause");
+	}
+
+	// Unblock the subseqent waiter
+	waiter->next->locked = false;
+}
+
+
+} // namespace spinlock
+} // namespace kutil