allow returns from checked::Resource.lock{,mut}

also add compiler barriers

Cargo.toml

 [package]
+authors = ["Jorge Aparicio <japaricious@gmail.com>"]
+build = "build.rs"
 name = "cortex-m-srp"
 version = "0.1.0"
-authors = ["Jorge Aparicio <japaricious@gmail.com>"]
 
 [dependencies]
 cortex-m = "0.2.0"

src/checked.rs

@@ -51,27 +51,33 @@ where
 
     /// Locks the resource, blocking tasks with priority equal or smaller than
     /// the ceiling `C`
-    pub fn lock<F>(&'static self, f: F)
+    pub fn lock<R, F>(&'static self, f: F) -> R
     where
-        F: FnOnce(&T),
+        F: FnOnce(&T) -> R,
     {
         unsafe {
             let old_basepri = acquire(&self.locked, C::ceiling());
-            f(&*self.data.get());
+            ::compiler_barrier();
+            let ret = f(&*self.data.get());
+            ::compiler_barrier();
             release(&self.locked, old_basepri);
+            ret
         }
     }
 
     /// Mutably locks the resource, blocking tasks with priority equal or
     /// smaller than the ceiling `C`
-    pub fn lock_mut<F>(&'static self, f: F)
+    pub fn lock_mut<R, F>(&'static self, f: F) -> R
         where
-        F: FnOnce(&mut T),
+        F: FnOnce(&mut T) -> R,
     {
         unsafe {
             let old_basepri = acquire(&self.locked, C::ceiling());
-            f(&mut *self.data.get());
+            ::compiler_barrier();
+            let ret = f(&mut *self.data.get());
+            ::compiler_barrier();
             release(&self.locked, old_basepri);
+            ret
         }
     }
 }

src/lib.rs

@@ -105,7 +105,9 @@ where
     lock_check(ceiling);
     let old_basepri = basepri::read();
     basepri_max::write(ceiling);
+    compiler_barrier();
     let ret = f(&*res, ptr::read(0 as *const _));
+    compiler_barrier();
     basepri::write(old_basepri);
     ret
 }
@@ -121,11 +123,23 @@ where
     lock_check(ceiling);
     let old_basepri = basepri::read();
     basepri_max::write(ceiling);
+    compiler_barrier();
     let ret = f(&mut *res, ptr::read(0 as *const _));
+    compiler_barrier();
     basepri::write(old_basepri);
     ret
 }
 
+fn compiler_barrier() {
+    unsafe {
+        asm!(""
+             :
+             :
+             : "memory"
+             : "volatile");
+    }
+}
+
 /// A peripheral as a resource
 pub struct ResourceP<P, Ceiling>
 where