Parker: park with callback before waiting

src/cachepadded.rs

@@ -146,9 +146,7 @@ use core::ops::{Deref, DerefMut};
     )),
     repr(align(64))
 )]
-pub struct CachePadded<T> {
+pub struct CachePadded<T>(pub T);
-    value: T,
-}
 
 unsafe impl<T: Send> Send for CachePadded<T> {}
 unsafe impl<T: Sync> Sync for CachePadded<T> {}
@@ -164,7 +162,7 @@ impl<T> CachePadded<T> {
     /// let padded_value = CachePadded::new(1);
     /// ```
     pub const fn new(t: T) -> CachePadded<T> {
-        CachePadded::<T> { value: t }
+        CachePadded::<T>(t)
     }
 
     /// Returns the inner value.
@@ -179,7 +177,7 @@ impl<T> CachePadded<T> {
     /// assert_eq!(value, 7);
     /// ```
     pub fn into_inner(self) -> T {
-        self.value
+        self.0
     }
 }
 
@@ -187,20 +185,20 @@ impl<T> Deref for CachePadded<T> {
     type Target = T;
 
     fn deref(&self) -> &T {
-        &self.value
+        &self.0
     }
 }
 
 impl<T> DerefMut for CachePadded<T> {
     fn deref_mut(&mut self) -> &mut T {
-        &mut self.value
+        &mut self.0
     }
 }
 
 impl<T: fmt::Debug> fmt::Debug for CachePadded<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("CachePadded")
-            .field("value", &self.value)
+            .field("value", &self.0)
             .finish()
     }
 }
@@ -213,6 +211,6 @@ impl<T> From<T> for CachePadded<T> {
 
 impl<T: fmt::Display> fmt::Display for CachePadded<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt::Display::fmt(&self.value, f)
+        fmt::Display::fmt(&self.0, f)
     }
 }

src/sync.rs

@@ -214,11 +214,38 @@ impl Parker {
     }
 
     pub fn park(&self) {
+        self.park_inner(|| ());
+    }
+
+    pub fn park_with_callback<F>(&self, before_sleep: F)
+    where
+        F: FnOnce(),
+    {
+        // This function is called when the thread is about to park.
+        self.park_inner(before_sleep);
+    }
+
+    // If the caller wants to park the thread on a mutex'd condition, it is
+    // possible for a deadlock to occur when the mutex is dropped just before
+    // this parker atomically changes it's state to `PARKED`. For that reason,
+    // we use a callback to allow the caller to perform any necessary actions
+    // before parking, but after the parker is set to `PARKED`. If another
+    // thread then checks the condition and checks if this thread should be
+    // woken, we will be immediately notified.
+    // Thusly, it is important that any caller synchronise any conditionals with
+    // a mutex externally, and then unlock that mutex in `before_sleep`.
+    fn park_inner<F>(&self, before_sleep: F)
+    where
+        F: FnOnce(),
+    {
         if self.mutex.fetch_sub(1, Ordering::Acquire) == Self::NOTIFIED {
+            before_sleep();
             // The thread was notified, so we can return immediately.
             return;
         }
 
+        before_sleep();
+
         loop {
             atomic_wait::wait(&self.mutex, Self::PARKED);