Add a few changes suggested by thomcc for the locks.

* Added needed compiler fences to `InitOnce::try_get`.
* Change the error in `RawMutex::raw_unlock` to better reflect the cause.

src/sync/locks.rs

@@ -40,7 +40,7 @@ impl RawMutex {
   fn raw_unlock(&self) {
     compiler_fence(Ordering::Release);
     if !self.0.replace(false) {
-      already_locked()
+      panic!("Internal error: Attempt to unlock a `RawMutex` which is not locked.")
     }
   }
 
@@ -158,28 +158,37 @@ impl<T> InitOnce<T> {
   /// returns `Ok`. If it returns `Err`, it will be called again in the
   /// future until an attempt at initialization succeeds.
   ///
-  /// Take care when sharing an `InitOnce` object between an IRQ and normal
-  /// code. If this function is called in an IRQ when it is already currently
-  /// being initialized by user code, this function will panic.
+  /// This function will disable interrupts if the contents are not already
+  /// initialized, which may cause audio skipping and similar issues. This
+  /// is generally not an issue as that will only happen once during the
+  /// lifetime of the program.
   pub fn try_get<E>(&self, initializer: impl FnOnce() -> Result<T, E>) -> Result<&T, E> {
     unsafe {
       if self.state.read() != 2 {
-        // Locks the initializer
-        if self.state.replace(1) != 0 {
-          panic!("Attempt to initialize `InitOnce` that is already in initialization.");
-        }
-
-        // Initialize the actual value.
-        let init = match initializer() {
-          Ok(v) => v,
-          Err(e) => {
-            assert_eq!(self.state.replace(0), 1);
-            return Err(e);
+        // We can afford to disable interrupts here, because this code path
+        // should only be called once for each variable.
+        with_irqs_disabled(|| -> Result<(), E> {
+          // Locks the initializer
+          if self.state.replace(1) != 0 {
+            panic!("Attempt to initialize `InitOnce` that is already in initialization.");
           }
-        };
-        ptr::write_volatile((*self.value.get()).as_mut_ptr(), init);
-        assert_eq!(self.state.replace(2), 1);
+
+          // Initialize the actual value.
+          let init = match initializer() {
+            Ok(v) => v,
+            Err(e) => {
+              assert_eq!(self.state.replace(0), 1);
+              return Err(e);
+            }
+          };
+          ptr::write_volatile((*self.value.get()).as_mut_ptr(), init);
+          compiler_fence(Ordering::Release);
+          assert_eq!(self.state.replace(2), 1);
+
+          Ok(())
+        });
       }
+      compiler_fence(Ordering::Acquire);
       Ok(&*(*self.value.get()).as_mut_ptr())
     }
   }