Upgrade lock_api to 0.4.0

Cargo.toml

@@ -13,4 +13,4 @@ repository = "https://github.com/rust-osdev/spinning_top"
 nightly = ["lock_api/nightly"]
 
 [dependencies]
-lock_api = "0.3.3"
+lock_api = "0.4.0"

src/spinlock.rs

@@ -18,7 +18,7 @@ use lock_api::{GuardSend, RawMutex};
 /// let lock = spinning_top::RawSpinlock::INIT;
 /// assert_eq!(lock.try_lock(), true); // lock it
 /// assert_eq!(lock.try_lock(), false); // can't be locked a second time
-/// lock.unlock(); // unlock it
+/// unsafe { lock.unlock(); } // unlock it
 /// assert_eq!(lock.try_lock(), true); // now it can be locked again
 #[derive(Debug)]
 pub struct RawSpinlock {
@@ -38,10 +38,7 @@ unsafe impl RawMutex for RawSpinlock {
         while !self.try_lock() {
             // Wait until the lock looks unlocked before retrying
             // Code from https://github.com/mvdnes/spin-rs/commit/d3e60d19adbde8c8e9d3199c7c51e51ee5a20bf6
-            //
-            // Relaxed ordering is fine here because the actual synchronization happens in
-            // the outer `while` loop.
-            while self.locked.load(Ordering::Relaxed) {
+            while self.is_locked() {
                 // Tell the CPU that we're inside a busy-wait loop
                 spin_loop_hint();
             }
@@ -63,9 +60,14 @@ unsafe impl RawMutex for RawSpinlock {
             .is_ok()
     }
 
-    fn unlock(&self) {
+    unsafe fn unlock(&self) {
         self.locked.store(false, Ordering::Release);
     }
+
+    fn is_locked(&self) -> bool {
+        // Relaxed is sufficient because this operation does not provide synchronization, only atomicity.
+        self.locked.load(Ordering::Relaxed)
+    }
 }
 
 /// A mutual exclusion (Mutex) type based on busy-waiting.