Merge #219

219: Use AtomicPtr for race::OnceRef to avoid ptr-int-ptr r=matklad a=Imberflur

Same rationale as #185

Potentially, a `OncePtr` abstraction could be factored out and used in both `OnceBox` and `OnceRef`.

Co-authored-by: Imbris <[email protected]>

src/race.rs

@@ -24,10 +24,11 @@ use atomic_polyfill as atomic;
 #[cfg(not(feature = "critical-section"))]
 use core::sync::atomic;
 
-use atomic::{AtomicUsize, Ordering};
+use atomic::{AtomicPtr, AtomicUsize, Ordering};
 use core::cell::UnsafeCell;
 use core::marker::PhantomData;
 use core::num::NonZeroUsize;
+use core::ptr;
 
 /// A thread-safe cell which can be written to only once.
 #[derive(Default, Debug)]
@@ -176,7 +177,7 @@ impl OnceBool {
 
 /// A thread-safe cell which can be written to only once.
 pub struct OnceRef<'a, T> {
-    inner: OnceNonZeroUsize,
+    inner: AtomicPtr<T>,
     ghost: PhantomData<UnsafeCell<&'a T>>,
 }
 
@@ -198,21 +199,27 @@ impl<'a, T> Default for OnceRef<'a, T> {
 impl<'a, T> OnceRef<'a, T> {
     /// Creates a new empty cell.
     pub const fn new() -> OnceRef<'a, T> {
-        OnceRef { inner: OnceNonZeroUsize::new(), ghost: PhantomData }
+        OnceRef { inner: AtomicPtr::new(ptr::null_mut()), ghost: PhantomData }
     }
 
     /// Gets a reference to the underlying value.
     pub fn get(&self) -> Option<&'a T> {
-        self.inner.get().map(|ptr| unsafe { &*(ptr.get() as *const T) })
+        let ptr = self.inner.load(Ordering::Acquire);
+        unsafe { ptr.as_ref() }
     }
 
     /// Sets the contents of this cell to `value`.
     ///
     /// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
     /// full.
     pub fn set(&self, value: &'a T) -> Result<(), ()> {
-        let ptr = NonZeroUsize::new(value as *const T as usize).unwrap();
-        self.inner.set(ptr)
+        let ptr = value as *const T as *mut T;
+        let exchange =
+            self.inner.compare_exchange(ptr::null_mut(), ptr, Ordering::AcqRel, Ordering::Acquire);
+        match exchange {
+            Ok(_) => Ok(()),
+            Err(_) => Err(()),
+        }
     }
 
     /// Gets the contents of the cell, initializing it with `f` if the cell was
@@ -225,9 +232,11 @@ impl<'a, T> OnceRef<'a, T> {
     where
         F: FnOnce() -> &'a T,
     {
-        let f = || NonZeroUsize::new(f() as *const T as usize).unwrap();
-        let ptr = self.inner.get_or_init(f);
-        unsafe { &*(ptr.get() as *const T) }
+        enum Void {}
+        match self.get_or_try_init(|| Ok::<&'a T, Void>(f())) {
+            Ok(val) => val,
+            Err(void) => match void {},
+        }
     }
 
     /// Gets the contents of the cell, initializing it with `f` if
@@ -241,9 +250,23 @@ impl<'a, T> OnceRef<'a, T> {
     where
         F: FnOnce() -> Result<&'a T, E>,
     {
-        let f = || f().map(|value| NonZeroUsize::new(value as *const T as usize).unwrap());
-        let ptr = self.inner.get_or_try_init(f)?;
-        unsafe { Ok(&*(ptr.get() as *const T)) }
+        let mut ptr = self.inner.load(Ordering::Acquire);
+
+        if ptr.is_null() {
+            // TODO replace with `cast_mut` when MSRV reaches 1.65.0 (also in `set`)
+            ptr = f()? as *const T as *mut T;
+            let exchange = self.inner.compare_exchange(
+                ptr::null_mut(),
+                ptr,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            );
+            if let Err(old) = exchange {
+                ptr = old;
+            }
+        }
+
+        Ok(unsafe { &*ptr })
     }
 
     /// ```compile_fail