Merge #696

696: Add async Mutex. r=Dirbaio a=Dirbaio

What it says on the tin :) 

It allows sharing data between tasks when you want to `.await` stuff while holding it locked.

Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>

examples/nrf/src/bin/mutex.rs

@@ -0,0 +1,44 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use defmt::{info, unwrap};
+use embassy::blocking_mutex::raw::ThreadModeRawMutex;
+use embassy::executor::Spawner;
+use embassy::mutex::Mutex;
+use embassy::time::{Duration, Timer};
+use embassy_nrf::Peripherals;
+
+use defmt_rtt as _; // global logger
+use panic_probe as _;
+
+static MUTEX: Mutex<ThreadModeRawMutex, u32> = Mutex::new(0);
+
+#[embassy::task]
+async fn my_task() {
+    loop {
+        {
+            let mut m = MUTEX.lock().await;
+            info!("start long operation");
+            *m += 1000;
+
+            // Hold the mutex for a long time.
+            Timer::after(Duration::from_secs(1)).await;
+            info!("end long operation: count = {}", *m);
+        }
+
+        Timer::after(Duration::from_secs(1)).await;
+    }
+}
+
+#[embassy::main]
+async fn main(spawner: Spawner, _p: Peripherals) {
+    unwrap!(spawner.spawn(my_task()));
+
+    loop {
+        Timer::after(Duration::from_millis(300)).await;
+        let mut m = MUTEX.lock().await;
+        *m += 1;
+        info!("short operation: count = {}", *m);
+    }
+}