Merge pull request #265 from TDHolmes/cas-fixup

Add cas support for risc-v via atomic-polyfill and fix mpmc/pool documentation

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Added `OldestOrdered` iterator for `HistoryBuffer`
 
+### Changed
+
+- `atomic-polyfill` is now enabled and used for `cas` atomic emulation on `riscv` targets
+
 ## [v0.7.9] - 2021-12-16
 
 ### Fixed

Cargo.toml

@@ -33,6 +33,12 @@ scoped_threadpool = "0.1.8"
 [target.thumbv6m-none-eabi.dependencies]
 atomic-polyfill = { version = "0.1.2", optional = true }
 
+[target.riscv32i-unknown-none-elf.dependencies]
+atomic-polyfill = { version = "0.1.4", optional = true }
+
+[target.riscv32imc-unknown-none-elf.dependencies]
+atomic-polyfill = { version = "0.1.4", optional = true }
+
 [dependencies]
 hash32 = "0.2.1"
 
@@ -58,3 +64,6 @@ version = "0.1"
 [dependencies.defmt]
 version = ">=0.2.0,<0.4"
 optional = true
+
+[package.metadata.docs.rs]
+all-features = true

build.rs

@@ -21,10 +21,19 @@ fn main() -> Result<(), Box<dyn Error>> {
         println!("cargo:rustc-cfg=armv7a");
     }
 
-    // built-in targets with no atomic / CAS support as of nightly-2019-12-17
+    // built-in targets with no atomic / CAS support as of nightly-2022-01-13
+    // AND not supported by the atomic-polyfill crate
     // see the `no-atomics.sh` / `no-cas.sh` script sitting next to this file
     match &target[..] {
-        "msp430-none-elf" | "riscv32i-unknown-none-elf" | "riscv32imc-unknown-none-elf" => {}
+        "avr-unknown-gnu-atmega328"
+        | "bpfeb-unknown-none"
+        | "bpfel-unknown-none"
+        | "msp430-none-elf"
+        // | "riscv32i-unknown-none-elf"    // supported by atomic-polyfill
+        // | "riscv32imc-unknown-none-elf"  // supported by atomic-polyfill
+        | "thumbv4t-none-eabi"
+        // | "thumbv6m-none-eabi"           // supported by atomic-polyfill
+         => {}
 
         _ => {
             println!("cargo:rustc-cfg=has_cas");
@@ -32,12 +41,30 @@ fn main() -> Result<(), Box<dyn Error>> {
     };
 
     match &target[..] {
-        "msp430-none-elf" | "riscv32i-unknown-none-elf" | "riscv32imc-unknown-none-elf" => {}
+        "avr-unknown-gnu-atmega328"
+        | "msp430-none-elf"
+        // | "riscv32i-unknown-none-elf"    // supported by atomic-polyfill
+        // | "riscv32imc-unknown-none-elf"  // supported by atomic-polyfill
+        => {}
 
         _ => {
             println!("cargo:rustc-cfg=has_atomics");
         }
     };
 
+    // Let the code know if it should use atomic-polyfill or not, and what aspects
+    // of polyfill it requires
+    match &target[..] {
+        "riscv32i-unknown-none-elf" | "riscv32imc-unknown-none-elf" => {
+            println!("cargo:rustc-cfg=full_atomic_polyfill");
+            println!("cargo:rustc-cfg=cas_atomic_polyfill");
+        }
+
+        "thumbv6m-none-eabi" => {
+            println!("cargo:rustc-cfg=cas_atomic_polyfill");
+        }
+        _ => {}
+    }
+
     Ok(())
 }

src/mpmc.rs

@@ -1,6 +1,7 @@
 //! A fixed capacity Multiple-Producer Multiple-Consumer (MPMC) lock-free queue
 //!
-//! NOTE: This module is not available on targets that do *not* support CAS operations, e.g. ARMv6-M
+//! NOTE: This module is not available on targets that do *not* support CAS operations and are not
+//! emulated by the [`atomic_polyfill`] crate (e.g., MSP430).
 //!
 //! # Example
 //!
@@ -73,8 +74,10 @@
 //!
 //! # Portability
 //!
-//! This module is not exposed to architectures that lack the instructions to implement CAS loops.
-//! Those architectures include ARMv6-M (`thumbv6m-none-eabi`) and MSP430 (`msp430-none-elf`).
+//! This module requires CAS atomic instructions which are not available on all architectures
+//! (e.g.  ARMv6-M (`thumbv6m-none-eabi`) and MSP430 (`msp430-none-elf`)). These atomics can be emulated
+//! however with [`atomic_polyfill`], which is enabled with the `cas` feature and is enabled by default
+//! for `thumbv6m-none-eabi` and `riscv32` targets. MSP430 is currently not supported by [`atomic_polyfill`].
 //!
 //! # References
 //!
@@ -84,18 +87,18 @@
 
 use core::{cell::UnsafeCell, mem::MaybeUninit};
 
-#[cfg(all(feature = "mpmc_large", not(armv6m)))]
+#[cfg(all(feature = "mpmc_large", not(cas_atomic_polyfill)))]
 type AtomicTargetSize = core::sync::atomic::AtomicUsize;
-#[cfg(all(feature = "mpmc_large", armv6m))]
+#[cfg(all(feature = "mpmc_large", cas_atomic_polyfill))]
 type AtomicTargetSize = atomic_polyfill::AtomicUsize;
-#[cfg(all(not(feature = "mpmc_large"), not(armv6m)))]
+#[cfg(all(not(feature = "mpmc_large"), not(cas_atomic_polyfill)))]
 type AtomicTargetSize = core::sync::atomic::AtomicU8;
-#[cfg(all(not(feature = "mpmc_large"), armv6m))]
+#[cfg(all(not(feature = "mpmc_large"), cas_atomic_polyfill))]
 type AtomicTargetSize = atomic_polyfill::AtomicU8;
 
-#[cfg(not(armv6m))]
+#[cfg(not(cas_atomic_polyfill))]
 type Ordering = core::sync::atomic::Ordering;
-#[cfg(armv6m)]
+#[cfg(cas_atomic_polyfill)]
 type Ordering = atomic_polyfill::Ordering;
 
 #[cfg(feature = "mpmc_large")]

src/pool/llsc.rs

@@ -3,10 +3,10 @@
 pub use core::ptr::NonNull as Ptr;
 use core::{cell::UnsafeCell, ptr};
 
-#[cfg(armv6m)]
+#[cfg(cas_atomic_polyfill)]
 use atomic_polyfill::{AtomicPtr, Ordering};
 
-#[cfg(not(armv6m))]
+#[cfg(not(cas_atomic_polyfill))]
 use core::sync::atomic::{AtomicPtr, Ordering};
 
 /// Unfortunate implementation detail required to use the

src/pool/mod.rs

@@ -1,6 +1,7 @@
 //! A heap-less, interrupt-safe, lock-free memory pool (\*)
 //!
-//! NOTE: This module is not available on targets that do *not* support CAS operations, e.g. ARMv6-M
+//! NOTE: This module is not available on targets that do *not* support CAS operations and are not
+//! emulated by the [`atomic_polyfill`] crate (e.g., MSP430).
 //!
 //! (\*) Currently, the implementation is only lock-free *and* `Sync` on ARMv6, ARMv7-{A,R,M} & ARMv8-M
 //! devices
@@ -59,8 +60,10 @@
 //! on the target architecture (see section on ['Soundness'](#soundness) for more information). For
 //! this reason, `Pool` only implements `Sync` when compiling for some ARM cores.
 //!
-//! Also note that ARMv6-M architecture lacks the primitives for CAS loops so this module does *not*
-//! exist for `thumbv6m-none-eabi`.
+//! This module requires CAS atomic instructions which are not available on all architectures
+//! (e.g.  ARMv6-M (`thumbv6m-none-eabi`) and MSP430 (`msp430-none-elf`)). These atomics can be emulated
+//! however with [`atomic_polyfill`], which is enabled with the `cas` feature and is enabled by default
+//! for `thumbv6m-none-eabi` and `riscv32` targets. MSP430 is currently not supported by [`atomic_polyfill`].
 //!
 //! # Soundness
 //!

src/pool/singleton/arc.rs

@@ -81,10 +81,10 @@ use core::{
     sync::atomic,
 };
 
-#[cfg(armv6m)]
+#[cfg(cas_atomic_polyfill)]
 use atomic_polyfill::{AtomicUsize, Ordering};
 
-#[cfg(not(armv6m))]
+#[cfg(not(cas_atomic_polyfill))]
 use core::sync::atomic::{AtomicUsize, Ordering};
 
 use crate::pool::{self, stack::Ptr, Node};

src/spsc.rs

@@ -2,8 +2,8 @@
 //!
 //! Implementation based on <https://www.codeproject.com/Articles/43510/Lock-Free-Single-Producer-Single-Consumer-Circular>
 //!
-//! NOTE: This module is not available on targets that do *not* support atomic loads, e.g. RISC-V
-//! cores w/o the A (Atomic) extension
+//! NOTE: This module is not available on targets that do *not* support atomic loads and are not
+//! supported by [`atomic_polyfill`]. (e.g., MSP430).
 //!
 //! # Examples
 //!
@@ -84,13 +84,12 @@
 //! - The numbers reported correspond to the successful path (i.e. `Some` is returned by `dequeue`
 //! and `Ok` is returned by `enqueue`).
 
-use core::{
-    cell::UnsafeCell,
-    fmt, hash,
-    mem::MaybeUninit,
-    ptr,
-    sync::atomic::{AtomicUsize, Ordering},
-};
+use core::{cell::UnsafeCell, fmt, hash, mem::MaybeUninit, ptr};
+
+#[cfg(full_atomic_polyfill)]
+use atomic_polyfill::{AtomicUsize, Ordering};
+#[cfg(not(full_atomic_polyfill))]
+use core::sync::atomic::{AtomicUsize, Ordering};
 
 /// A statically allocated single producer single consumer queue with a capacity of `N - 1` elements
 ///