3 files changed, 213 insertions, 2 deletions

diff --git a/embassy/src/lib.rs b/embassy/src/lib.rs
index 6b24b5989..ec697b40a 100644
--- a/embassy/src/lib.rs
+++ b/embassy/src/lib.rs
@@ -10,15 +10,15 @@ pub(crate) mod fmt;
 
 pub mod blocking_mutex;
 pub mod channel;
-pub mod waitqueue;
-
 pub mod executor;
 #[cfg(cortex_m)]
 pub mod interrupt;
 pub mod io;
+pub mod mutex;
 #[cfg(feature = "time")]
 pub mod time;
 pub mod util;
+pub mod waitqueue;
 
 #[cfg(feature = "nightly")]
 pub use embassy_macros::{main, task};
diff --git a/embassy/src/mutex.rs b/embassy/src/mutex.rs
new file mode 100644
index 000000000..27353bd4d
--- /dev/null
+++ b/embassy/src/mutex.rs
@@ -0,0 +1,167 @@
+/// Async mutex.
+///
+/// The mutex is generic over a blocking [`RawMutex`](crate::blocking_mutex::raw::RawMutex).
+/// The raw mutex is used to guard access to the internal "is locked" flag. It
+/// is held for very short periods only, while locking and unlocking. It is *not* held
+/// for the entire time the async Mutex is locked.
+use core::cell::{RefCell, UnsafeCell};
+use core::ops::{Deref, DerefMut};
+use core::task::Poll;
+use futures::future::poll_fn;
+
+use crate::blocking_mutex::raw::RawMutex;
+use crate::blocking_mutex::Mutex as BlockingMutex;
+use crate::waitqueue::WakerRegistration;
+
+/// Error returned by [`Mutex::try_lock`]
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub struct TryLockError;
+
+struct State {
+    locked: bool,
+    waker: WakerRegistration,
+}
+
+pub struct Mutex<M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    state: BlockingMutex<M, RefCell<State>>,
+    inner: UnsafeCell<T>,
+}
+
+unsafe impl<M: RawMutex + Send, T: ?Sized + Send> Send for Mutex<M, T> {}
+unsafe impl<M: RawMutex + Sync, T: ?Sized + Send> Sync for Mutex<M, T> {}
+
+/// Async mutex.
+impl<M, T> Mutex<M, T>
+where
+    M: RawMutex,
+{
+    /// Create a new mutex with the given value.
+    #[cfg(feature = "nightly")]
+    pub const fn new(value: T) -> Self {
+        Self {
+            inner: UnsafeCell::new(value),
+            state: BlockingMutex::new(RefCell::new(State {
+                locked: false,
+                waker: WakerRegistration::new(),
+            })),
+        }
+    }
+
+    /// Create a new mutex with the given value.
+    #[cfg(not(feature = "nightly"))]
+    pub fn new(value: T) -> Self {
+        Self {
+            inner: UnsafeCell::new(value),
+            state: BlockingMutex::new(RefCell::new(State {
+                locked: false,
+                waker: WakerRegistration::new(),
+            })),
+        }
+    }
+}
+
+impl<M, T> Mutex<M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    /// Lock the mutex.
+    ///
+    /// This will wait for the mutex to be unlocked if it's already locked.
+    pub async fn lock(&self) -> MutexGuard<'_, M, T> {
+        poll_fn(|cx| {
+            let ready = self.state.lock(|s| {
+                let mut s = s.borrow_mut();
+                if s.locked {
+                    s.waker.register(cx.waker());
+                    false
+                } else {
+                    s.locked = true;
+                    true
+                }
+            });
+
+            if ready {
+                Poll::Ready(MutexGuard { mutex: self })
+            } else {
+                Poll::Pending
+            }
+        })
+        .await
+    }
+
+    /// Attempt to immediately lock the mutex.
+    ///
+    /// If the mutex is already locked, this will return an error instead of waiting.
+    pub fn try_lock(&self) -> Result<MutexGuard<'_, M, T>, TryLockError> {
+        self.state.lock(|s| {
+            let mut s = s.borrow_mut();
+            if s.locked {
+                Err(TryLockError)
+            } else {
+                s.locked = true;
+                Ok(())
+            }
+        })?;
+
+        Ok(MutexGuard { mutex: self })
+    }
+}
+
+/// Async mutex guard.
+///
+/// Owning an instance of this type indicates having
+/// successfully locked the mutex, and grants access to the contents.
+///
+/// Dropping it unlocks the mutex.
+pub struct MutexGuard<'a, M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    mutex: &'a Mutex<M, T>,
+}
+
+impl<'a, M, T> Drop for MutexGuard<'a, M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    fn drop(&mut self) {
+        self.mutex.state.lock(|s| {
+            let mut s = s.borrow_mut();
+            s.locked = false;
+            s.waker.wake();
+        })
+    }
+}
+
+impl<'a, M, T> Deref for MutexGuard<'a, M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        // Safety: the MutexGuard represents exclusive access to the contents
+        // of the mutex, so it's OK to get it.
+        unsafe { &*(self.mutex.inner.get() as *const T) }
+    }
+}
+
+impl<'a, M, T> DerefMut for MutexGuard<'a, M, T>
+where
+    M: RawMutex,
+    T: ?Sized,
+{
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // Safety: the MutexGuard represents exclusive access to the contents
+        // of the mutex, so it's OK to get it.
+        unsafe { &mut *(self.mutex.inner.get()) }
+    }
+}
diff --git a/examples/nrf/src/bin/mutex.rs b/examples/nrf/src/bin/mutex.rs
new file mode 100644
index 000000000..db1b72f6d
--- /dev/null
+++ b/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);
+    }
+}