Add alternate implementations of synchronization primitives

.github/workflows/ci.yml

@@ -29,8 +29,14 @@ jobs:
         if: startsWith(matrix.rust, 'nightly')
         run: cargo check -Z features=dev_dep
       - run: cargo test
+      - run: cargo test --features portable-atomic
       - name: Install cargo-hack
         uses: taiki-e/install-action@cargo-hack
+      - name: Run with Loom enabled
+        run: cargo test --test loom --features loom
+        env:
+          RUSTFLAGS: ${{ env.RUSTFLAGS }} --cfg loom
+          LOOM_MAX_PREEMPTIONS: 2
       - run: rustup target add thumbv7m-none-eabi
       - run: cargo hack build --target thumbv7m-none-eabi --no-default-features --no-dev-deps
 
@@ -46,6 +52,7 @@ jobs:
       - name: Install Rust
         run: rustup update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
       - run: cargo build
+      - run: cargo build --features portable-atomic
       - name: Install cargo-hack
         uses: taiki-e/install-action@cargo-hack
       - run: rustup target add thumbv7m-none-eabi

Cargo.toml

@@ -19,6 +19,12 @@ bench = false
 
 [dependencies]
 crossbeam-utils = { version = "0.8.11", default-features = false }
+portable-atomic = { version = "0.3", default-features = false, optional = true }
+
+# Enables loom testing. This feature is permanently unstable and the API may
+# change at any time.
+[target.'cfg(loom)'.dependencies]
+loom = { version = "0.5", optional = true }
 
 [[bench]]
 name = "bench"

src/bounded.rs

@@ -1,10 +1,12 @@
 use alloc::{boxed::Box, vec::Vec};
-use core::cell::UnsafeCell;
 use core::mem::MaybeUninit;
-use core::sync::atomic::{AtomicUsize, Ordering};
 
 use crossbeam_utils::CachePadded;
 
+use crate::sync::atomic::{AtomicUsize, Ordering};
+use crate::sync::cell::UnsafeCell;
+#[allow(unused_imports)]
+use crate::sync::prelude::*;
 use crate::{busy_wait, PopError, PushError};
 
 /// A slot in a queue.
@@ -118,9 +120,9 @@ impl<T> Bounded<T> {
                 ) {
                     Ok(_) => {
                         // Write the value into the slot and update the stamp.
-                        unsafe {
-                            slot.value.get().write(MaybeUninit::new(value));
-                        }
+                        slot.value.with_mut(|slot| unsafe {
+                            slot.write(MaybeUninit::new(value));
+                        });
                         slot.stamp.store(tail + 1, Ordering::Release);
                         return Ok(());
                     }
@@ -138,6 +140,10 @@ impl<T> Bounded<T> {
                     return Err(PushError::Full(value));
                 }
 
+                // Loom complains if there isn't an explicit busy wait here.
+                #[cfg(loom)]
+                busy_wait();
+
                 tail = self.tail.load(Ordering::Relaxed);
             } else {
                 // Yield because we need to wait for the stamp to get updated.
@@ -181,7 +187,9 @@ impl<T> Bounded<T> {
                 ) {
                     Ok(_) => {
                         // Read the value from the slot and update the stamp.
-                        let value = unsafe { slot.value.get().read().assume_init() };
+                        let value = slot
+                            .value
+                            .with_mut(|slot| unsafe { slot.read().assume_init() });
                         slot.stamp
                             .store(head.wrapping_add(self.one_lap), Ordering::Release);
                         return Ok(value);
@@ -204,6 +212,10 @@ impl<T> Bounded<T> {
                     }
                 }
 
+                // Loom complains if there isn't a busy-wait here.
+                #[cfg(loom)]
+                busy_wait();
+
                 head = self.head.load(Ordering::Relaxed);
             } else {
                 // Yield because we need to wait for the stamp to get updated.
@@ -284,37 +296,48 @@ impl<T> Bounded<T> {
 impl<T> Drop for Bounded<T> {
     fn drop(&mut self) {
         // Get the index of the head.
-        let head = *self.head.get_mut();
-        let tail = *self.tail.get_mut();
-
-        let hix = head & (self.mark_bit - 1);
-        let tix = tail & (self.mark_bit - 1);
-
-        let len = if hix < tix {
-            tix - hix
-        } else if hix > tix {
-            self.buffer.len() - hix + tix
-        } else if (tail & !self.mark_bit) == head {
-            0
-        } else {
-            self.buffer.len()
-        };
-
-        // Loop over all slots that hold a value and drop them.
-        for i in 0..len {
-            // Compute the index of the next slot holding a value.
-            let index = if hix + i < self.buffer.len() {
-                hix + i
-            } else {
-                hix + i - self.buffer.len()
-            };
+        let Self {
+            head,
+            tail,
+            buffer,
+            mark_bit,
+            ..
+        } = self;
 
-            // Drop the value in the slot.
-            let slot = &self.buffer[index];
-            unsafe {
-                let value = &mut *slot.value.get();
-                value.as_mut_ptr().drop_in_place();
-            }
-        }
+        let mark_bit = *mark_bit;
+
+        head.with_mut(|&mut head| {
+            tail.with_mut(|&mut tail| {
+                let hix = head & (mark_bit - 1);
+                let tix = tail & (mark_bit - 1);
+
+                let len = if hix < tix {
+                    tix - hix
+                } else if hix > tix {
+                    buffer.len() - hix + tix
+                } else if (tail & !mark_bit) == head {
+                    0
+                } else {
+                    buffer.len()
+                };
+
+                // Loop over all slots that hold a value and drop them.
+                for i in 0..len {
+                    // Compute the index of the next slot holding a value.
+                    let index = if hix + i < buffer.len() {
+                        hix + i
+                    } else {
+                        hix + i - buffer.len()
+                    };
+
+                    // Drop the value in the slot.
+                    let slot = &buffer[index];
+                    slot.value.with_mut(|slot| unsafe {
+                        let value = &mut *slot;
+                        value.as_mut_ptr().drop_in_place();
+                    });
+                }
+            });
+        });
     }
 }

src/lib.rs

@@ -26,14 +26,23 @@
 //!
 //! # Features
 //!
-//! `concurrent-queue` used an `std` default feature. With this feature enabled, this crate will
+//! `concurrent-queue` uses an `std` default feature. With this feature enabled, this crate will
 //! use [`std::thread::yield_now`] to avoid busy waiting in tight loops. However, with this
 //! feature disabled, [`core::hint::spin_loop`] will be used instead. Disabling `std` will allow
 //! this crate to be used on `no_std` platforms at the potential expense of more busy waiting.
 //!
+//! There is also a `portable-atomic` feature, which uses a polyfill from the
+//! [`portable-atomic`] crate to provide atomic operations on platforms that do not support them.
+//! See the [`README`] for the [`portable-atomic`] crate for more information on how to use it on
+//! single-threaded targets. Note that even with this feature enabled, `concurrent-queue` still
+//! requires a global allocator to be available. See the documentation for the
+//! [`std::alloc::GlobalAlloc`] trait for more information.
+//!
 //! [Bounded]: `ConcurrentQueue::bounded()`
 //! [Unbounded]: `ConcurrentQueue::unbounded()`
 //! [closed]: `ConcurrentQueue::close()`
+//! [`portable-atomic`]: https://crates.io/crates/portable-atomic
+//! [`README`]: https://github.com/taiki-e/portable-atomic/blob/main/README.md#optional-cfg
 
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
 #![no_std]
@@ -44,7 +53,7 @@ extern crate std;
 
 use alloc::boxed::Box;
 use core::fmt;
-use core::sync::atomic::{self, AtomicUsize, Ordering};
+use sync::atomic::{self, AtomicUsize, Ordering};
 
 #[cfg(feature = "std")]
 use std::error;
@@ -53,12 +62,15 @@ use std::panic::{RefUnwindSafe, UnwindSafe};
 
 use crate::bounded::Bounded;
 use crate::single::Single;
+use crate::sync::busy_wait;
 use crate::unbounded::Unbounded;
 
 mod bounded;
 mod single;
 mod unbounded;
 
+mod sync;
+
 /// A concurrent queue.
 ///
 /// # Examples
@@ -463,24 +475,13 @@ impl<T> fmt::Display for PushError<T> {
     }
 }
 
-/// Notify the CPU that we are currently busy-waiting.
-#[inline]
-fn busy_wait() {
-    #[cfg(feature = "std")]
-    std::thread::yield_now();
-    // Use the deprecated `spin_loop_hint` here in order to
-    // avoid bumping the MSRV.
-    #[allow(deprecated)]
-    #[cfg(not(feature = "std"))]
-    core::sync::atomic::spin_loop_hint()
-}
-
 /// Equivalent to `atomic::fence(Ordering::SeqCst)`, but in some cases faster.
 #[inline]
 fn full_fence() {
     if cfg!(all(
         any(target_arch = "x86", target_arch = "x86_64"),
-        not(miri)
+        not(miri),
+        not(loom)
     )) {
         // HACK(stjepang): On x86 architectures there are two different ways of executing
         // a `SeqCst` fence.

src/single.rs

@@ -1,7 +1,9 @@
-use core::cell::UnsafeCell;
 use core::mem::MaybeUninit;
-use core::sync::atomic::{AtomicUsize, Ordering};
 
+use crate::sync::atomic::{AtomicUsize, Ordering};
+use crate::sync::cell::UnsafeCell;
+#[allow(unused_imports)]
+use crate::sync::prelude::*;
 use crate::{busy_wait, PopError, PushError};
 
 const LOCKED: usize = 1 << 0;
@@ -33,7 +35,9 @@ impl<T> Single<T> {
 
         if state == 0 {
             // Write the value and unlock.
-            unsafe { self.slot.get().write(MaybeUninit::new(value)) }
+            self.slot.with_mut(|slot| unsafe {
+                slot.write(MaybeUninit::new(value));
+            });
             self.state.fetch_and(!LOCKED, Ordering::Release);
             Ok(())
         } else if state & CLOSED != 0 {
@@ -60,7 +64,9 @@ impl<T> Single<T> {
 
             if prev == state {
                 // Read the value and unlock.
-                let value = unsafe { self.slot.get().read().assume_init() };
+                let value = self
+                    .slot
+                    .with_mut(|slot| unsafe { slot.read().assume_init() });
                 self.state.fetch_and(!LOCKED, Ordering::Release);
                 return Ok(value);
             }
@@ -118,11 +124,14 @@ impl<T> Single<T> {
 impl<T> Drop for Single<T> {
     fn drop(&mut self) {
         // Drop the value in the slot.
-        if *self.state.get_mut() & PUSHED != 0 {
-            unsafe {
-                let value = &mut *self.slot.get();
-                value.as_mut_ptr().drop_in_place();
+        let Self { state, slot } = self;
+        state.with_mut(|state| {
+            if *state & PUSHED != 0 {
+                slot.with_mut(|slot| unsafe {
+                    let value = &mut *slot;
+                    value.as_mut_ptr().drop_in_place();
+                });
             }
-        }
+        });
     }
 }