From e0034b6e9d7d15bcd5a5b9990e31d26c887846d5 Mon Sep 17 00:00:00 2001
From: Emil Fresk <emil.fresk@gmail.com>
Date: Tue, 21 Apr 2020 21:14:43 +0200
Subject: [PATCH 1/4] Added arch specific mutex implementation

---
 Cargo.toml   |  1 +
 src/lib.rs   |  1 +
 src/mutex.rs | 25 +++++++++++++++++++++++++
 3 files changed, 27 insertions(+)
 create mode 100644 src/mutex.rs

diff --git a/Cargo.toml b/Cargo.toml
index 5137dce5..b1c61141 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -18,6 +18,7 @@ links = "cortex-m"  # prevent multiple versions of this crate to be linked toget
 [dependencies]
 bare-metal = { version = "0.2.0", features = ["const-fn"] }
 volatile-register = "0.2.0"
+mutex-trait = "0.2.0"
 
 [features]
 cm7-r0p1 = []
diff --git a/src/lib.rs b/src/lib.rs
index f8b56060..52870413 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -62,5 +62,6 @@ pub mod interrupt;
 pub mod itm;
 pub mod peripheral;
 pub mod register;
+pub mod mutex;
 
 pub use crate::peripheral::Peripherals;
diff --git a/src/mutex.rs b/src/mutex.rs
new file mode 100644
index 00000000..7346e354
--- /dev/null
+++ b/src/mutex.rs
@@ -0,0 +1,25 @@
+//! Implementation of a critical section based mutex that also implements the `mutex-trait`.
+
+use core::cell::RefCell;
+
+/// A critical section based mutex
+pub struct CriticalSectionMutex<T> {
+    data: RefCell<T>,
+}
+
+impl<T> CriticalSectionMutex<T> {
+    /// Create a new mutex
+    pub const fn new(data: T) -> Self {
+        CriticalSectionMutex {
+            data: RefCell::new(data),
+        }
+    }
+}
+
+impl<T> mutex_trait::Mutex for &'_ CriticalSectionMutex<T> {
+    type Data = T;
+
+    fn lock<R>(&mut self, f: impl FnOnce(&mut Self::Data) -> R) -> R {
+        crate::interrupt::free(|_| f(&mut *self.data.borrow_mut()))
+    }
+}

From 64ff1e185001fdb3d56b7fc166db08713e0d97d2 Mon Sep 17 00:00:00 2001
From: Emil Fresk <emil.fresk@gmail.com>
Date: Tue, 21 Apr 2020 21:56:47 +0200
Subject: [PATCH 2/4] UnsafeCell test

---
 src/mutex.rs | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/src/mutex.rs b/src/mutex.rs
index 7346e354..69485120 100644
--- a/src/mutex.rs
+++ b/src/mutex.rs
@@ -1,17 +1,17 @@
 //! Implementation of a critical section based mutex that also implements the `mutex-trait`.
 
-use core::cell::RefCell;
+use core::cell::UnsafeCell;
 
 /// A critical section based mutex
 pub struct CriticalSectionMutex<T> {
-    data: RefCell<T>,
+    data: UnsafeCell<T>,
 }
 
 impl<T> CriticalSectionMutex<T> {
     /// Create a new mutex
     pub const fn new(data: T) -> Self {
         CriticalSectionMutex {
-            data: RefCell::new(data),
+            data: UnsafeCell::new(data),
         }
     }
 }
@@ -20,6 +20,11 @@ impl<T> mutex_trait::Mutex for &'_ CriticalSectionMutex<T> {
     type Data = T;
 
     fn lock<R>(&mut self, f: impl FnOnce(&mut Self::Data) -> R) -> R {
-        crate::interrupt::free(|_| f(&mut *self.data.borrow_mut()))
+        crate::interrupt::free(|_| f(unsafe { &mut *self.data.get() }))
     }
 }
+
+// NOTE A `Mutex` can be used as a channel so the protected data must be `Send`
+// to prevent sending non-Sendable stuff (e.g. access tokens) across different
+// execution contexts (e.g. interrupts)
+unsafe impl<T> Sync for CriticalSectionMutex<T> where T: Send {}

From b4635839218108d4c68158ad38ca26e063137f79 Mon Sep 17 00:00:00 2001
From: Emil Fresk <emil.fresk@gmail.com>
Date: Tue, 21 Apr 2020 21:57:13 +0200
Subject: [PATCH 3/4] Update src/mutex.rs

Co-Authored-By: Jonas Schievink <jonasschievink@gmail.com>
---
 src/mutex.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/mutex.rs b/src/mutex.rs
index 69485120..86bd5d55 100644
--- a/src/mutex.rs
+++ b/src/mutex.rs
@@ -2,7 +2,7 @@
 
 use core::cell::UnsafeCell;
 
-/// A critical section based mutex
+/// A critical section based mutex.
 pub struct CriticalSectionMutex<T> {
     data: UnsafeCell<T>,
 }

From 2dca8c948f47a1c23e9f33bff26721f5a33f2fd8 Mon Sep 17 00:00:00 2001
From: Emil Fresk <emil.fresk@gmail.com>
Date: Tue, 21 Apr 2020 22:03:48 +0200
Subject: [PATCH 4/4] More docs

---
 src/mutex.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/mutex.rs b/src/mutex.rs
index 86bd5d55..0975b83c 100644
--- a/src/mutex.rs
+++ b/src/mutex.rs
@@ -1,4 +1,8 @@
 //! Implementation of a critical section based mutex that also implements the `mutex-trait`.
+//!
+//! ## Safety
+//!
+//! Note that this is only safe in single core applications.
 
 use core::cell::UnsafeCell;