feat(threads): add mutex with priority inheritance

Cargo.toml

@@ -27,6 +27,7 @@ members = [
   "tests/i2c-controller",
   "tests/threading-dynamic-prios",
   "tests/threading-lock",
+  "tests/threading-mutex",
 ]
 
 exclude = ["src/lib"]

src/riot-rs-threads/src/lib.rs

@@ -24,6 +24,7 @@
 #![cfg_attr(not(test), no_std)]
 #![feature(naked_functions)]
 #![feature(used_with_arg)]
+#![feature(negative_impls)]
 #![cfg_attr(target_arch = "xtensa", feature(asm_experimental_arch))]
 // Disable indexing lints for now, possible panics are documented or rely on internally-enforced
 // invariants

src/riot-rs-threads/src/sync/mod.rs

@@ -1,6 +1,8 @@
 //! Synchronization primitives.
 mod channel;
 mod lock;
+mod mutex;
 
 pub use channel::Channel;
 pub use lock::Lock;
+pub use mutex::{Mutex, MutexGuard};

src/riot-rs-threads/src/sync/mutex.rs

@@ -0,0 +1,205 @@
+use core::{
+    cell::UnsafeCell,
+    ops::{Deref, DerefMut},
+};
+
+use critical_section::CriticalSection;
+use riot_rs_runqueue::{RunqueueId, ThreadId};
+
+use crate::{thread::ThreadState, threadlist::ThreadList, THREADS};
+
+/// A basic mutex with priority inheritance.
+pub struct Mutex<T> {
+    state: UnsafeCell<LockState>,
+    inner: UnsafeCell<T>,
+}
+
+/// State of a [`Mutex`].
+enum LockState {
+    Unlocked,
+    Locked {
+        /// The current owner of the lock.
+        owner_id: ThreadId,
+        /// The original priority of the current owner (without priority inheritance).
+        owner_prio: RunqueueId,
+        //. Waiters for the mutex.
+        waiters: ThreadList,
+    },
+}
+
+impl LockState {
+    /// Returns a [`LockState::Locked`] with the current thread as the owner
+    /// and an empty waitlist.
+    ///
+    /// # Panics
+    ///
+    /// Panics if called outside of a thread context.
+    fn locked_with_current(cs: CriticalSection) -> Self {
+        let (owner_id, owner_prio) = THREADS.with_mut_cs(cs, |mut threads| {
+            let current = threads
+                .current()
+                .expect("Function should be called inside a thread context.");
+            (current.pid, current.prio)
+        });
+        LockState::Locked {
+            waiters: ThreadList::new(),
+            owner_id,
+            owner_prio,
+        }
+    }
+}
+
+impl<T> Mutex<T> {
+    /// Creates a new **unlocked** [`Mutex`].
+    pub const fn new(value: T) -> Self {
+        Self {
+            state: UnsafeCell::new(LockState::Unlocked),
+            inner: UnsafeCell::new(value),
+        }
+    }
+}
+
+impl<T> Mutex<T> {
+    /// Returns whether the mutex is locked.
+    pub fn is_locked(&self) -> bool {
+        critical_section::with(|_| {
+            let state = unsafe { &*self.state.get() };
+            !matches!(state, LockState::Unlocked)
+        })
+    }
+
+    /// Acquires a mutex, blocking the current thread until it is able to do so.
+    ///
+    /// If the mutex was unlocked, it will be locked and a [`MutexGuard`] is returned.
+    /// If the mutex is locked, this function will block the current thread until the mutex gets
+    /// unlocked elsewhere.
+    ///
+    /// If the current owner of the mutex has a lower priority than the current thread, it will inherit
+    /// the waiting thread's priority.
+    /// The priority is reset once the mutex is released. This means that a **user can not change a thread's
+    /// priority while it holds the lock**, because it will be changed back after release!
+    ///
+    /// # Panics
+    ///
+    /// Panics if called outside of a thread context.
+    pub fn lock(&self) -> MutexGuard<T> {
+        critical_section::with(|cs| {
+            // SAFETY: access to the state only happens in critical sections, so it's always unique.
+            let state = unsafe { &mut *self.state.get() };
+            match state {
+                LockState::Unlocked => {
+                    *state = LockState::locked_with_current(cs);
+                }
+                LockState::Locked {
+                    waiters,
+                    owner_id,
+                    owner_prio,
+                } => {
+                    // Insert thread in waitlist, which also triggers the scheduler.
+                    match waiters.put_current(cs, ThreadState::LockBlocked) {
+                        // `Some` when the inserted thread is the highest priority
+                        // thread in the waitlist.
+                        Some(waiter_prio) if waiter_prio > *owner_prio => {
+                            // Current mutex owner inherits the priority.
+                            THREADS.with_mut_cs(cs, |mut threads| {
+                                threads.set_priority(*owner_id, waiter_prio)
+                            });
+                        }
+                        _ => {}
+                    }
+                    // Context switch happens here as soon as we leave the critical section.
+                }
+            }
+        });
+        // Mutex was either directly acquired because it was unlocked, or the current thread was entered
+        // to the waitlist. In the latter case, it only continues running here after it was popped again
+        // from the waitlist and the thread acquired the mutex.
+
+        MutexGuard { mutex: self }
+    }
+
+    /// Attempts to acquire this lock, in a non-blocking fashion.
+    ///
+    /// If the mutex was unlocked, it will be locked and a [`MutexGuard`] is returned.
+    /// If the mutex was locked `None` is returned.
+    pub fn try_lock(&self) -> Option<MutexGuard<T>> {
+        critical_section::with(|cs| {
+            // SAFETY: access to the state only happens in critical sections, so it's always unique.
+            let state = unsafe { &mut *self.state.get() };
+            if let LockState::Unlocked = *state {
+                *state = LockState::locked_with_current(cs);
+                Some(MutexGuard { mutex: self })
+            } else {
+                None
+            }
+        })
+    }
+
+    /// Releases the mutex.
+    ///
+    /// If there are waiters, the first waiter will be woken up.
+    fn release(&self) {
+        critical_section::with(|cs| {
+            // SAFETY: access to the state only happens in critical sections, so it's always unique.
+            let state = unsafe { &mut *self.state.get() };
+            if let LockState::Locked {
+                waiters,
+                owner_id,
+                owner_prio,
+            } = state
+            {
+                // Reset original priority of owner.
+                THREADS.with_mut_cs(cs, |mut threads| {
+                    threads.set_priority(*owner_id, *owner_prio)
+                });
+                // Pop next thread from waitlist so that it can acquire the mutex.
+                if let Some((pid, _)) = waiters.pop(cs) {
+                    THREADS.with_mut_cs(cs, |threads| {
+                        *owner_id = pid;
+                        *owner_prio = threads.get_unchecked(pid).prio;
+                    })
+                } else {
+                    // Unlock if waitlist was empty.
+                    *state = LockState::Unlocked
+                }
+            }
+        })
+    }
+}
+
+unsafe impl<T> Sync for Mutex<T> {}
+
+/// Grants access to the [`Mutex`] inner data.
+///
+/// Dropping the [`MutexGuard`] will unlock the [`Mutex`];
+pub struct MutexGuard<'a, T> {
+    mutex: &'a Mutex<T>,
+}
+
+impl<'a, T> Deref for MutexGuard<'a, T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: MutexGuard always has unique access.
+        unsafe { &*self.mutex.inner.get() }
+    }
+}
+
+impl<'a, T> DerefMut for MutexGuard<'a, T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // SAFETY: MutexGuard always has unique access.
+        unsafe { &mut *self.mutex.inner.get() }
+    }
+}
+
+impl<'a, T> Drop for MutexGuard<'a, T> {
+    fn drop(&mut self) {
+        // Unlock the mutex when the guard is dropped.
+        self.mutex.release()
+    }
+}
+
+// The [`MutexGuard`] is tied to a thread, it must not be possible to `Send` it to another thread.
+impl<T> !Send for MutexGuard<'_, T> {}
+
+unsafe impl<T: Sync> Sync for MutexGuard<'_, T> {}

src/riot-rs-threads/src/threadlist.rs

@@ -1,12 +1,12 @@
 use critical_section::CriticalSection;
 
-use crate::{thread::Thread, ThreadId, ThreadState, THREADS};
+use crate::{thread::Thread, RunqueueId, ThreadId, ThreadState, THREADS};
 
 /// Manages blocked [`super::Thread`]s for a resource, and triggering the scheduler when needed.
 #[derive(Debug, Default)]
 pub struct ThreadList {
     /// Next thread to run once the resource is available.
-    pub head: Option<ThreadId>,
+    head: Option<ThreadId>,
 }
 
 impl ThreadList {
@@ -17,10 +17,12 @@ impl ThreadList {
 
     /// Puts the current (blocked) thread into this [`ThreadList`] and triggers the scheduler.
     ///
+    /// Returns a `RunqueueId` if the highest priority among the waiters in the list has changed.
+    ///
     /// # Panics
     ///
     /// Panics if this is called outside of a thread context.
-    pub fn put_current(&mut self, cs: CriticalSection, state: ThreadState) {
+    pub fn put_current(&mut self, cs: CriticalSection, state: ThreadState) -> Option<RunqueueId> {
         THREADS.with_mut_cs(cs, |mut threads| {
             let &mut Thread { pid, prio, .. } = threads
                 .current()
@@ -35,12 +37,19 @@ impl ThreadList {
                 next = threads.thread_blocklist[usize::from(n)];
             }
             threads.thread_blocklist[usize::from(pid)] = next;
-            match curr {
-                Some(curr) => threads.thread_blocklist[usize::from(curr)] = Some(pid),
-                _ => self.head = Some(pid),
-            }
+            let inherit_priority = match curr {
+                Some(curr) => {
+                    threads.thread_blocklist[usize::from(curr)] = Some(pid);
+                    None
+                }
+                None => {
+                    self.head = Some(pid);
+                    Some(prio)
+                }
+            };
             threads.set_state(pid, state);
-        });
+            inherit_priority
+        })
     }
 
     /// Removes the head from this [`ThreadList`].

tests/laze.yml

@@ -6,3 +6,4 @@ subdirs:
   - i2c-controller
   - threading-dynamic-prios
   - threading-lock
+  - threading-mutex

tests/threading-mutex/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "threading-mutex"
+version = "0.1.0"
+authors = ["Elena Frank <elena.frank@proton.me>"]
+license.workspace = true
+edition.workspace = true
+publish = false
+
+[dependencies]
+embassy-executor = { workspace = true }
+riot-rs = { path = "../../src/riot-rs", features = ["threading"] }
+riot-rs-boards = { path = "../../src/riot-rs-boards" }
+portable-atomic = "1.6.0"

tests/threading-mutex/laze.yml

@@ -0,0 +1,5 @@
+apps:
+  - name: threading-mutex
+    selects:
+      - ?release
+      - sw/threading