From 5abb33765144c95bce0c297e8a8d343f194cf774 Mon Sep 17 00:00:00 2001
From: Jorge Aparicio <jorge@japaric.io>
Date: Mon, 5 Nov 2018 00:29:16 +0100
Subject: [PATCH 1/3] 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

From 691206d7633e1e5d43546bf6316ab5f8ee90e3ac Mon Sep 17 00:00:00 2001
From: Jorge Aparicio <jorge@japaric.io>
Date: Mon, 5 Nov 2018 00:50:19 +0100
Subject: [PATCH 2/3] Relaxed is OK if the target is a single-core device

for a multi-core device AcqRel ordering for the fetch-add operation and Release
ordering for the store operation would have been required for correctness
---
 src/concurrency/concurrency.md | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/src/concurrency/concurrency.md b/src/concurrency/concurrency.md
index a6f888a..d4fb352 100644
--- a/src/concurrency/concurrency.md
+++ b/src/concurrency/concurrency.md
@@ -192,13 +192,12 @@ 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
-operations like incrementing a counter, where we are not synchronising any
-other tasks on the counter, `Relaxed` is sufficient, and will have the best
-performance on typical embedded platforms. Stricter ordering will cause the
-compiler to emit memory barriers around the atomic operations; depending on
-what you're using atomics for you may or may not need this! The precise
-details of the atomic model are complicated and best described elsewhere.
+instructions, and also has consequences on cache visibility. Assuming that the
+target is a single core platform `Relaxed` is sufficient and the most efficient
+choice in this particular case. Stricter ordering will cause the compiler to
+emit memory barriers around the atomic operations; depending on what you're
+using atomics for you may or may not need this! The precise details of the
+atomic model are complicated and best described elsewhere.
 
 For more details on atomics and ordering, see the [nomicon].
 

From e8e8862dec07128907ba84e225bfbfae8d63f623 Mon Sep 17 00:00:00 2001
From: Jorge Aparicio <jorge@japaric.io>
Date: Mon, 5 Nov 2018 01:41:06 +0100
Subject: [PATCH 3/3] fix paths

---
 src/concurrency/concurrency.md | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/concurrency/concurrency.md b/src/concurrency/concurrency.md
index d4fb352..45b28ff 100644
--- a/src/concurrency/concurrency.md
+++ b/src/concurrency/concurrency.md
@@ -174,7 +174,7 @@ fn main() -> ! {
         if state && !last_state {
             last_state = state;
             // Use `fetch_add` to atomically add 1 to COUNTER
-            COUNTER.fetch_add(1, atomic::Ordering::Relaxed);
+            COUNTER.fetch_add(1, Ordering::Relaxed);
         }
     }
 }
@@ -182,7 +182,7 @@ fn main() -> ! {
 #[interrupt]
 fn timer() {
     // Use `store` to write 0 directly to COUNTER
-    COUNTER.store(0, atomic::Ordering::Relaxed)
+    COUNTER.store(0, Ordering::Relaxed)
 }
 ```