tests: Instrument with loom

This should hopefully allow us to get to the bottom of these deadlocks.

Signed-off-by: John Nunley <dev@notgull.net>

.github/workflows/ci.yml

@@ -78,6 +78,18 @@ jobs:
         run: rustup update stable
       - run: cargo clippy --all-features --all-targets
 
+  loom:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Rust
+        run: rustup update stable
+      - name: Loom tests
+        run: cargo test --release --test loom --features loom
+        env:
+          RUSTFLAGS: "--cfg=loom"
+          LOOM_MAX_PREEMPTIONS: 4
+
   fmt:
     runs-on: ubuntu-latest
     steps:

Cargo.toml

@@ -18,10 +18,25 @@ exclude = ["/.*"]
 event-listener = { version = "5.0.0", default-features = false }
 event-listener-strategy = { version = "0.5.0", default-features = false }
 pin-project-lite = "0.2.11"
+portable-atomic-util = { version = "0.1.4", default-features = false, optional = true, features = ["alloc"] }
+
+[dependencies.portable_atomic_crate]
+package = "portable-atomic"
+version = "1.2.0"
+default-features = false
+optional = true
+
+[target.'cfg(loom)'.dependencies]
+loom = { version = "0.7", optional = true }
 
 [features]
 default = ["std"]
+portable-atomic = ["portable-atomic-util", "portable_atomic_crate"]
 std = ["event-listener/std", "event-listener-strategy/std"]
+loom = ["event-listener/loom", "dep:loom"]
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(loom)'] }
 
 [dev-dependencies]
 fastrand = "2.0.0"

src/barrier.rs

@@ -23,28 +23,31 @@ struct State {
 }
 
 impl Barrier {
-    /// Creates a barrier that can block the given number of tasks.
-    ///
-    /// A barrier will block `n`-1 tasks which call [`wait()`] and then wake up all tasks
-    /// at once when the `n`th task calls [`wait()`].
-    ///
-    /// [`wait()`]: `Barrier::wait()`
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use async_lock::Barrier;
-    ///
-    /// let barrier = Barrier::new(5);
-    /// ```
-    pub const fn new(n: usize) -> Barrier {
-        Barrier {
-            n,
-            state: Mutex::new(State {
-                count: 0,
-                generation_id: 0,
-            }),
-            event: Event::new(),
+    const_fn! {
+        const_if: #[cfg(not(loom))];
+        /// Creates a barrier that can block the given number of tasks.
+        ///
+        /// A barrier will block `n`-1 tasks which call [`wait()`] and then wake up all tasks
+        /// at once when the `n`th task calls [`wait()`].
+        ///
+        /// [`wait()`]: `Barrier::wait()`
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use async_lock::Barrier;
+        ///
+        /// let barrier = Barrier::new(5);
+        /// ```
+        pub const fn new(n: usize) -> Barrier {
+            Barrier {
+                n,
+                state: Mutex::new(State {
+                    count: 0,
+                    generation_id: 0,
+                }),
+                event: Event::new(),
+            }
         }
     }
 

src/lib.rs

@@ -71,6 +71,22 @@ macro_rules! pin {
     }
 }
 
+/// Make the given function const if the given condition is true.
+macro_rules! const_fn {
+    (
+        const_if: #[cfg($($cfg:tt)+)];
+        $(#[$($attr:tt)*])*
+        $vis:vis const fn $($rest:tt)*
+    ) => {
+        #[cfg($($cfg)+)]
+        $(#[$($attr)*])*
+        $vis const fn $($rest)*
+        #[cfg(not($($cfg)+))]
+        $(#[$($attr)*])*
+        $vis fn $($rest)*
+    };
+}
+
 mod barrier;
 mod mutex;
 mod once_cell;
@@ -97,6 +113,38 @@ pub mod futures {
     pub use crate::semaphore::{Acquire, AcquireArc};
 }
 
+#[cfg(not(loom))]
+/// Synchronization primitive implementation.
+mod sync {
+    pub(super) use core::sync::atomic;
+
+    pub(super) trait WithMut {
+        type Output;
+
+        fn with_mut<F, R>(&mut self, f: F) -> R
+        where
+            F: FnOnce(&mut Self::Output) -> R;
+    }
+
+    impl WithMut for atomic::AtomicUsize {
+        type Output = usize;
+
+        #[inline]
+        fn with_mut<F, R>(&mut self, f: F) -> R
+        where
+            F: FnOnce(&mut Self::Output) -> R,
+        {
+            f(self.get_mut())
+        }
+    }
+}
+
+#[cfg(loom)]
+/// Synchronization primitive implementation.
+mod sync {
+    pub(super) use loom::sync::atomic;
+}
+
 #[cold]
 fn abort() -> ! {
     // For no_std targets, panicking while panicking is defined as an abort

src/mutex.rs

@@ -4,12 +4,14 @@ use core::fmt;
 use core::marker::{PhantomData, PhantomPinned};
 use core::ops::{Deref, DerefMut};
 use core::pin::Pin;
-use core::sync::atomic::{AtomicUsize, Ordering};
 use core::task::Poll;
 use core::usize;
 
 use alloc::sync::Arc;
 
+// We don't use loom::UnsafeCell as that doesn't work with the Mutex API.
+use crate::sync::atomic::{AtomicUsize, Ordering};
+
 #[cfg(all(feature = "std", not(target_family = "wasm")))]
 use std::time::{Duration, Instant};
 
@@ -56,20 +58,23 @@ unsafe impl<T: Send + ?Sized> Send for Mutex<T> {}
 unsafe impl<T: Send + ?Sized> Sync for Mutex<T> {}
 
 impl<T> Mutex<T> {
-    /// Creates a new async mutex.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use async_lock::Mutex;
-    ///
-    /// let mutex = Mutex::new(0);
-    /// ```
-    pub const fn new(data: T) -> Mutex<T> {
-        Mutex {
-            state: AtomicUsize::new(0),
-            lock_ops: Event::new(),
-            data: UnsafeCell::new(data),
+    const_fn! {
+        const_if: #[cfg(not(loom))];
+        /// Creates a new async mutex.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use async_lock::Mutex;
+        ///
+        /// let mutex = Mutex::new(0);
+        /// ```
+        pub const fn new(data: T) -> Mutex<T> {
+            Mutex {
+                state: AtomicUsize::new(0),
+                lock_ops: Event::new(),
+                data: UnsafeCell::new(data),
+            }
         }
     }
 
@@ -186,7 +191,7 @@ impl<T: ?Sized> Mutex<T> {
     /// # })
     /// ```
     pub fn get_mut(&mut self) -> &mut T {
-        unsafe { &mut *self.data.get() }
+        self.data.get_mut()
     }
 
     /// Unlocks the mutex directly.

src/once_cell.rs

@@ -4,7 +4,11 @@ use core::fmt;
 use core::future::Future;
 use core::mem::{forget, MaybeUninit};
 use core::ptr;
-use core::sync::atomic::{AtomicUsize, Ordering};
+
+use crate::sync::atomic::{AtomicUsize, Ordering};
+
+#[cfg(not(loom))]
+use crate::sync::WithMut;
 
 #[cfg(all(feature = "std", not(target_family = "wasm")))]
 use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
@@ -107,22 +111,25 @@ unsafe impl<T: Send> Send for OnceCell<T> {}
 unsafe impl<T: Send + Sync> Sync for OnceCell<T> {}
 
 impl<T> OnceCell<T> {
-    /// Create a new, uninitialized `OnceCell`.
-    ///
-    /// # Example
-    ///
-    /// ```rust
-    /// use async_lock::OnceCell;
-    ///
-    /// let cell = OnceCell::new();
-    /// # cell.set_blocking(1);
-    /// ```
-    pub const fn new() -> Self {
-        Self {
-            active_initializers: Event::new(),
-            passive_waiters: Event::new(),
-            state: AtomicUsize::new(State::Uninitialized as _),
-            value: UnsafeCell::new(MaybeUninit::uninit()),
+    const_fn! {
+        const_if: #[cfg(not(loom))];
+        /// Create a new, uninitialized `OnceCell`.
+        ///
+        /// # Example
+        ///
+        /// ```rust
+        /// use async_lock::OnceCell;
+        ///
+        /// let cell = OnceCell::new();
+        /// # cell.set_blocking(1);
+        /// ```
+        pub const fn new() -> Self {
+            Self {
+                active_initializers: Event::new(),
+                passive_waiters: Event::new(),
+                state: AtomicUsize::new(State::Uninitialized as _),
+                value: UnsafeCell::new(MaybeUninit::uninit()),
+            }
         }
     }
 
@@ -194,13 +201,15 @@ impl<T> OnceCell<T> {
     /// # });
     /// ```
     pub fn get_mut(&mut self) -> Option<&mut T> {
-        if State::from(*self.state.get_mut()) == State::Initialized {
-            // SAFETY: We know that the value is initialized, so it is safe to
-            // read it.
-            Some(unsafe { &mut *self.value.get().cast() })
-        } else {
-            None
-        }
+        self.state.with_mut(|state| {
+            if State::from(*state) == State::Initialized {
+                // SAFETY: We know that the value is initialized, so it is safe to
+                // read it.
+                Some(unsafe { &mut *self.value.get().cast() })
+            } else {
+                None
+            }
+        })
     }
 
     /// Take the value out of this `OnceCell`, moving it back to the uninitialized
@@ -219,15 +228,17 @@ impl<T> OnceCell<T> {
     /// # });
     /// ```
     pub fn take(&mut self) -> Option<T> {
-        if State::from(*self.state.get_mut()) == State::Initialized {
-            // SAFETY: We know that the value is initialized, so it is safe to
-            // read it.
-            let value = unsafe { ptr::read(self.value.get().cast()) };
-            *self.state.get_mut() = State::Uninitialized.into();
-            Some(value)
-        } else {
-            None
-        }
+        self.state.with_mut(|state| {
+            if State::from(*state) == State::Initialized {
+                // SAFETY: We know that the value is initialized, so it is safe to
+                // read it.
+                let value = unsafe { ptr::read(self.value.get().cast()) };
+                *state = State::Uninitialized.into();
+                Some(value)
+            } else {
+                None
+            }
+        })
     }
 
     /// Convert this `OnceCell` into the inner value, if it is initialized.
@@ -754,11 +765,13 @@ impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
 
 impl<T> Drop for OnceCell<T> {
     fn drop(&mut self) {
-        if State::from(*self.state.get_mut()) == State::Initialized {
-            // SAFETY: We know that the value is initialized, so it is safe to
-            // drop it.
-            unsafe { self.value.get().cast::<T>().drop_in_place() }
-        }
+        self.state.with_mut(|state| {
+            if State::from(*state) == State::Initialized {
+                // SAFETY: We know that the value is initialized, so it is safe to
+                // drop it.
+                unsafe { self.value.get().cast::<T>().drop_in_place() }
+            }
+        });
     }
 }
 

src/rwlock.rs

@@ -55,21 +55,24 @@ unsafe impl<T: Send + ?Sized> Send for RwLock<T> {}
 unsafe impl<T: Send + Sync + ?Sized> Sync for RwLock<T> {}
 
 impl<T> RwLock<T> {
-    /// Creates a new reader-writer lock.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use async_lock::RwLock;
-    ///
-    /// let lock = RwLock::new(0);
-    /// ```
-    #[must_use]
-    #[inline]
-    pub const fn new(t: T) -> RwLock<T> {
-        RwLock {
-            raw: RawRwLock::new(),
-            value: UnsafeCell::new(t),
+    const_fn! {
+        const_if: #[cfg(not(loom))];
+        /// Creates a new reader-writer lock.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use async_lock::RwLock;
+        ///
+        /// let lock = RwLock::new(0);
+        /// ```
+        #[must_use]
+        #[inline]
+        pub const fn new(t: T) -> RwLock<T> {
+            RwLock {
+                raw: RawRwLock::new(),
+                value: UnsafeCell::new(t),
+            }
         }
     }