From f3b9be7beede167295e8dc431e417bea77bb2455 Mon Sep 17 00:00:00 2001
From: Alexander van Saase <avsaase@gmail.com>
Date: Wed, 19 Mar 2025 19:32:05 +0100
Subject: [PATCH 1/2] embassy-sync: add lock_mut to blocking_mutex::Mutex

---
 embassy-sync/src/blocking_mutex/mod.rs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/embassy-sync/src/blocking_mutex/mod.rs b/embassy-sync/src/blocking_mutex/mod.rs
index beafdb43d..5b0e4144a 100644
--- a/embassy-sync/src/blocking_mutex/mod.rs
+++ b/embassy-sync/src/blocking_mutex/mod.rs
@@ -50,6 +50,20 @@ impl<R: RawMutex, T> Mutex<R, T> {
             f(inner)
         })
     }
+
+    /// Creates a critical section and grants temporary mutable access to the protected data.
+    ///
+    /// # Safety
+    /// This method is unsafe because calling this method when the mutex is already locked,
+    /// either using this method or `lock`, violates Rust's aliasing rules.
+    pub unsafe fn lock_mut<U>(&self, f: impl FnOnce(&mut T) -> U) -> U {
+        self.raw.lock(|| {
+            let ptr = self.data.get() as *mut T;
+            // Safety: we have exclusive access to the data, as long as this mutex is not locked re-entrantly
+            let inner = unsafe { &mut *ptr };
+            f(inner)
+        })
+    }
 }
 
 impl<R, T> Mutex<R, T> {

From 7a031eed66ef27e83b8582e7c1e7ca00d16ccf64 Mon Sep 17 00:00:00 2001
From: Alexander van Saase <avsaase@gmail.com>
Date: Fri, 21 Mar 2025 23:02:04 +0100
Subject: [PATCH 2/2] Add note about RefCell alternative

---
 embassy-sync/src/blocking_mutex/mod.rs | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/embassy-sync/src/blocking_mutex/mod.rs b/embassy-sync/src/blocking_mutex/mod.rs
index 5b0e4144a..a41bc3569 100644
--- a/embassy-sync/src/blocking_mutex/mod.rs
+++ b/embassy-sync/src/blocking_mutex/mod.rs
@@ -54,8 +54,11 @@ impl<R: RawMutex, T> Mutex<R, T> {
     /// Creates a critical section and grants temporary mutable access to the protected data.
     ///
     /// # Safety
-    /// This method is unsafe because calling this method when the mutex is already locked,
-    /// either using this method or `lock`, violates Rust's aliasing rules.
+    ///
+    /// This method is marked unsafe because calling this method re-entrantly, i.e. within
+    /// another `lock_mut` or `lock` closure, violates Rust's aliasing rules. Calling this
+    /// method at the same time from different tasks is safe. For a safe alternative with
+    /// mutable access that never causes UB, use a `RefCell` in a `Mutex`.
     pub unsafe fn lock_mut<U>(&self, f: impl FnOnce(&mut T) -> U) -> U {
         self.raw.lock(|| {
             let ptr = self.data.get() as *mut T;