From b1642ad065b04f452227bff58e951f67fb4cec47 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Thu, 17 Apr 2025 00:42:57 -0400
Subject: kernel: synch: Add lazy spinlock acquire

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 sys/include/sys/spinlock.h |  4 +++-
 sys/kern/kern_synch.c      | 26 ++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/sys/include/sys/spinlock.h b/sys/include/sys/spinlock.h
index c136e05..b416152 100644
--- a/sys/include/sys/spinlock.h
+++ b/sys/include/sys/spinlock.h
@@ -33,13 +33,15 @@
 #include <sys/types.h>
 
 struct spinlock {
-    volatile bool lock;
+    volatile int lock;
 };
 
 #if defined(_KERNEL)
 
 void spinlock_acquire(struct spinlock *lock);
 void spinlock_release(struct spinlock *lock);
+
+int spinlock_try_acquire(struct spinlock *lock);
 int spinlock_usleep(struct spinlock *lock, size_t usec_max);
 
 #endif
diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c
index 2011c61..2b64673 100644
--- a/sys/kern/kern_synch.c
+++ b/sys/kern/kern_synch.c
@@ -30,6 +30,7 @@
 #include <sys/types.h>
 #include <sys/systm.h>
 #include <sys/errno.h>
+#include <sys/atomic.h>
 #include <sys/syslog.h>
 #include <sys/spinlock.h>
 #include <dev/timer.h>
@@ -79,6 +80,31 @@ spinlock_acquire(struct spinlock *lock)
     while (__atomic_test_and_set(&lock->lock, __ATOMIC_ACQUIRE));
 }
 
+/*
+ * Lazy acquire a spinlock
+ *
+ * spinlock_try_acquire() may only spin one thread
+ * at a time, threads that want to spin too must
+ * explicity do it on their own.
+ *
+ * This function returns 1 (a value that may be
+ * spinned on) when the lock is acquired and a
+ * thread is already spinning on it.
+ */
+int
+spinlock_try_acquire(struct spinlock *lock)
+{
+    volatile int locked;
+
+    locked = atomic_load_int(&lock->lock);
+    if (locked != 0) {
+        return 1;
+    }
+
+    while (__atomic_test_and_set(&lock->lock, __ATOMIC_ACQUIRE));
+    return 0;
+}
+
 void
 spinlock_release(struct spinlock *lock)
 {
-- 
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