Use futex directly in waitval.

src/sync/waitval.rs

-// Semaphores need one post per wait, Once doesn't have separate post and wait, and while mutexes
-// wait for releasing the lock, it calls notify_one and needs locking again to access the value.
-
 use core::cell::UnsafeCell;
 use core::mem::MaybeUninit;
-use core::sync::atomic::Ordering;
+use core::sync::atomic::{AtomicU32 as AtomicUint, Ordering};
 
 use super::*;
 
+/// An unsafe "one thread to one thread" synchronization primitive. Used for and modeled after
+/// pthread_join only, at the moment.
 pub struct Waitval<T> {
-    state: AtomicLock,
+    state: AtomicUint,
     value: UnsafeCell<MaybeUninit<T>>,
 }
 
@@ -18,20 +17,22 @@ unsafe impl<T: Send + Sync> Sync for Waitval<T> {}
 impl<T> Waitval<T> {
     pub const fn new() -> Self {
         Self {
-            state: AtomicLock::new(0),
+            state: AtomicUint::new(0),
             value: UnsafeCell::new(MaybeUninit::uninit()),
         }
     }
 
+    // SAFETY: Caller must ensure both (1) that the value has not yet been initialized, and (2)
+    // that this is never run by more than one thread simultaneously.
     pub unsafe fn post(&self, value: T) {
         self.value.get().write(MaybeUninit::new(value));
         self.state.store(1, Ordering::Release);
-        self.state.notify_all();
+        crate::sync::futex_wake(&self.state, i32::MAX);
     }
 
     pub fn wait(&self) -> &T {
         while self.state.load(Ordering::Acquire) == 0 {
-            self.state.wait_if(0, None);
+            crate::sync::futex_wait(&self.state, 0, None);
         }
 
         unsafe { (*self.value.get()).assume_init_ref() }