4 files changed, 39 insertions, 4 deletions

diff --git a/embassy/Cargo.toml b/embassy/Cargo.toml
index 39ad64a0d..2f3ee453c 100644
--- a/embassy/Cargo.toml
+++ b/embassy/Cargo.toml
@@ -26,3 +26,5 @@ embassy-macros  = { version = "0.1.0", path = "../embassy-macros"}
 embassy-traits  = { version = "0.1.0", path = "../embassy-traits"}
 atomic-polyfill  = { version = "0.1.1" }
 
+# Workaround https://github.com/japaric/cast.rs/pull/27
+cast = { version = "=0.2.3", default-features = false }
diff --git a/embassy/src/util/critical_section.rs b/embassy/src/util/critical_section.rs
new file mode 100644
index 000000000..12ad6f20e
--- /dev/null
+++ b/embassy/src/util/critical_section.rs
@@ -0,0 +1,30 @@
+pub use cs::{critical_section, CriticalSection};
+
+#[cfg(feature = "std")]
+mod cs {
+    static INIT: std::sync::Once = std::sync::Once::new();
+    static mut BKL: Option<std::sync::Mutex<()>> = None;
+
+    pub type CriticalSection = std::sync::MutexGuard<'static, ()>;
+    pub fn critical_section<F, R>(f: F) -> R
+    where
+        F: FnOnce(&CriticalSection) -> R,
+    {
+        INIT.call_once(|| unsafe {
+            BKL.replace(std::sync::Mutex::new(()));
+        });
+        let guard = unsafe { BKL.as_ref().unwrap().lock().unwrap() };
+        f(&guard)
+    }
+}
+
+#[cfg(not(feature = "std"))]
+mod cs {
+    pub use cortex_m::interrupt::CriticalSection;
+    pub fn critical_section<F, R>(f: F) -> R
+    where
+        F: FnOnce(&CriticalSection) -> R,
+    {
+        cortex_m::interrupt::free(f)
+    }
+}
diff --git a/embassy/src/util/mod.rs b/embassy/src/util/mod.rs
index 9f82422dd..25b570cc6 100644
--- a/embassy/src/util/mod.rs
+++ b/embassy/src/util/mod.rs
@@ -5,6 +5,7 @@ mod mutex;
 mod on_drop;
 mod portal;
 mod signal;
+mod critical_section;
 
 #[cfg_attr(feature = "executor-agnostic", path = "waker_agnostic.rs")]
 mod waker;
@@ -16,6 +17,7 @@ pub use on_drop::*;
 pub use portal::*;
 pub use signal::*;
 pub use waker::*;
+pub use critical_section::*;
 
 pub trait PeripheralBorrow {
     type Target;
diff --git a/embassy/src/util/signal.rs b/embassy/src/util/signal.rs
index 2fc86906f..a62abcd55 100644
--- a/embassy/src/util/signal.rs
+++ b/embassy/src/util/signal.rs
@@ -11,6 +11,7 @@ use ptr::NonNull;
 use crate::executor;
 use crate::fmt::panic;
 use crate::interrupt::{Interrupt, InterruptExt};
+use crate::util::critical_section::critical_section;
 
 /// Synchronization primitive. Allows creating awaitable signals that may be passed between tasks.
 ///
@@ -37,7 +38,7 @@ impl<T: Send> Signal<T> {
 
     /// Mark this Signal as completed.
     pub fn signal(&self, val: T) {
-        cortex_m::interrupt::free(|_| unsafe {
+        critical_section(|_| unsafe {
             let state = &mut *self.state.get();
             if let State::Waiting(waker) = mem::replace(state, State::Signaled(val)) {
                 waker.wake();
@@ -46,14 +47,14 @@ impl<T: Send> Signal<T> {
     }
 
     pub fn reset(&self) {
-        cortex_m::interrupt::free(|_| unsafe {
+        critical_section(|_| unsafe {
             let state = &mut *self.state.get();
             *state = State::None
         })
     }
 
     pub fn poll_wait(&self, cx: &mut Context<'_>) -> Poll<T> {
-        cortex_m::interrupt::free(|_| unsafe {
+        critical_section(|_| unsafe {
             let state = &mut *self.state.get();
             match state {
                 State::None => {
@@ -77,7 +78,7 @@ impl<T: Send> Signal<T> {
 
     /// non-blocking method to check whether this signal has been signaled.
     pub fn signaled(&self) -> bool {
-        cortex_m::interrupt::free(|_| matches!(unsafe { &*self.state.get() }, State::Signaled(_)))
+        critical_section(|_| matches!(unsafe { &*self.state.get() }, State::Signaled(_)))
     }
 }