From 5abb33765144c95bce0c297e8a8d343f194cf774 Mon Sep 17 00:00:00 2001 From: Jorge Aparicio Date: Mon, 5 Nov 2018 00:29:16 +0100 Subject: [PATCH] concurrency: AtomicUsize doesn't need be in a static mut variable --- src/concurrency/concurrency.md | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/src/concurrency/concurrency.md b/src/concurrency/concurrency.md index a4d4671..a6f888a 100644 --- a/src/concurrency/concurrency.md +++ b/src/concurrency/concurrency.md @@ -161,9 +161,9 @@ increment operation. These atomic operations are safe even across multiple cores. ```rust -// New type for COUNTER -use core::sync::atomic; -static mut COUNTER: atomic::AtomicUsize = atomic::ATOMIC_USIZE_INIT; +use core::sync::atomic::{AtomicUsize, Ordering}; + +static COUNTER: AtomicUsize = AtomicUsize::new(0); #[entry] fn main() -> ! { @@ -174,7 +174,7 @@ fn main() -> ! { if state && !last_state { last_state = state; // Use `fetch_add` to atomically add 1 to COUNTER - unsafe { COUNTER.fetch_add(1, atomic::Ordering::Relaxed) }; + COUNTER.fetch_add(1, atomic::Ordering::Relaxed); } } } @@ -182,13 +182,14 @@ fn main() -> ! { #[interrupt] fn timer() { // Use `store` to write 0 directly to COUNTER - unsafe { COUNTER.store(0, atomic::Ordering::Relaxed) } + COUNTER.store(0, atomic::Ordering::Relaxed) } ``` -We still require `unsafe` blocks since `COUNTER` is a `static mut`, but we no -longer have the overhead of disabling all interrupts. When possible, this is a -better solution — but it may not be supported on your platform. +This time `COUNTER` is a safe `static` variable. Thanks to the `AtomicUsize` +type `COUNTER` can be safely modified from both the interrupt handler and the +main thread without disabling interrupts. When possible, this is a better +solution — but it may not be supported on your platform. A note on [`Ordering`]: this affects how the compiler and hardware may reorder instructions, and also has consequences on cache visibility. For simple atomic