revise Hermit's mutex interface to support the behaviour of StaticMutex

rust-lang#77147 simplifies things by splitting this Mutex type
into two types matching the two use cases: StaticMutex and MovableMutex.
To support the behavior of StaticMutex, we move part of the mutex
implementation into libstd.

Author: stlankes

library/std/src/sys/hermit/mutex.rs

@@ -1,43 +1,217 @@
+use crate::cell::UnsafeCell;
+use crate::collections::VecDeque;
 use crate::ffi::c_void;
+use crate::ops::{Deref, DerefMut, Drop};
 use crate::ptr;
+use crate::sync::atomic::{AtomicUsize, Ordering, spin_loop_hint};
 use crate::sys::hermit::abi;
 
+/// This type provides a lock based on busy waiting to realize mutual exclusion
+///
+/// # Description
+///
+/// This structure behaves a lot like a common mutex. There are some differences:
+///
+/// - By using busy waiting, it can be used outside the runtime.
+/// - It is a so called ticket lock (https://en.wikipedia.org/wiki/Ticket_lock)
+///   and completly fair.
+#[cfg_attr(target_arch = "x86_64", repr(align(128)))]
+#[cfg_attr(not(target_arch = "x86_64"), repr(align(64)))]
+struct Spinlock<T: ?Sized> {
+    queue: AtomicUsize,
+    dequeue: AtomicUsize,
+    data: UnsafeCell<T>,
+}
+
+unsafe impl<T: ?Sized + Send> Sync for Spinlock<T> {}
+unsafe impl<T: ?Sized + Send> Send for Spinlock<T> {}
+
+/// A guard to which the protected data can be accessed
+///
+/// When the guard falls out of scope it will release the lock.
+struct SpinlockGuard<'a, T: ?Sized + 'a> {
+    dequeue: &'a AtomicUsize,
+    data: &'a mut T,
+}
+
+impl<T> Spinlock<T> {
+    pub const fn new(user_data: T) -> Spinlock<T> {
+        SpinlockGuard { dequeue: &self.dequeue, data: &mut *self.data.get() }
+    }
+
+    #[inline]
+    fn obtain_lock(&self) {
+        let ticket = self.queue.fetch_add(1, Ordering::SeqCst) + 1;
+        while self.dequeue.load(Ordering::SeqCst) != ticket {
+            spin_loop_hint();
+        }
+    }
+
+    #[inline]
+    pub unsafe fn lock(&self) -> SpinlockGuard<'_, T> {
+        self.obtain_lock();
+        SpinlockGuard {
+            dequeue: &self.dequeue,
+            data: &mut *self.data.get(),
+        }
+    }
+}
+
+impl<T: ?Sized + Default> Default for Spinlock<T> {
+    fn default() -> Spinlock<T> {
+        Spinlock::new(Default::default())
+    }
+}
+
+impl<'a, T: ?Sized> Deref for SpinlockGuard<'a, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        &*self.data
+    }
+}
+
+impl<'a, T: ?Sized> DerefMut for SpinlockGuard<'a, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut *self.data
+    }
+}
+
+impl<'a, T: ?Sized> Drop for SpinlockGuard<'a, T> {
+    /// The dropping of the SpinlockGuard will release the lock it was created from.
+    fn drop(&mut self) {
+        self.dequeue.fetch_add(1, Ordering::SeqCst);
+    }
+}
+
+/// Realize a priority queue for tasks
+struct PriorityQueue {
+    queues: [Option<VecDeque<abi::Tid>>; abi::NO_PRIORITIES],
+    prio_bitmap: u64,
+}
+
+impl PriorityQueue {
+    pub const fn new() -> PriorityQueue {
+        PriorityQueue {
+            queues: [
+                None, None, None, None, None, None, None, None, None, None, None, None, None, None,
+                None, None, None, None, None, None, None, None, None, None, None, None, None, None,
+                None, None, None,
+            ],
+            prio_bitmap: 0,
+        }
+    }
+
+    /// Add a task handle by its priority to the queue
+    pub fn push(&mut self, prio: abi::Priority, id: abi::Tid) {
+        let i: usize = prio.into().into();
+        self.prio_bitmap |= (1 << i) as u64;
+        if let Some(queue) = &mut self.queues[i] {
+            queue.push_back(id);
+        } else {
+            let mut queue = VecDeque::new();
+            queue.push_back(id);
+            self.queues[i] = Some(queue);
+        }
+    }
+
+    fn pop_from_queue(&mut self, queue_index: usize) -> Option<abi::Tid> {
+        if let Some(queue) = &mut self.queues[queue_index] {
+            let id = queue.pop_front();
+
+            if queue.is_empty() {
+                self.prio_bitmap &= !(1 << queue_index as u64);
+            }
+
+            id
+        } else {
+            None
+        }
+    }
+
+    /// Pop the task handle with the highest priority from the queue
+    pub fn pop(&mut self) -> Option<abi::Tid> {
+        for i in 0..abi::NO_PRIORITIES {
+            if self.prio_bitmap & (1 << i) != 0 {
+                return self.pop_from_queue(i);
+            }
+        }
+
+        None
+    }
+}
+
+struct MutexInner {
+    locked: bool,
+    blocked_task: PriorityQueue,
+}
+
+impl MutexInner {
+    pub const fn new() -> MutexInner {
+        MutexInner {
+            locked: false,
+            blocked_task: PriorityQueue::new(),
+        }
+    }
+}
+
 pub struct Mutex {
-    inner: *const c_void,
+    inner: Spinlock<MutexInner>,
 }
 
 unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}
 
 impl Mutex {
     pub const fn new() -> Mutex {
-        Mutex { inner: ptr::null() }
+        Mutex {
+            inner: Spinlock::new(MutexInner::new()),
+        }
     }
 
     #[inline]
     pub unsafe fn init(&mut self) {
-        let _ = abi::sem_init(&mut self.inner as *mut *const c_void, 1);
+        self.inner = Spinlock::new(MutexInner::new());
     }
 
     #[inline]
     pub unsafe fn lock(&self) {
-        let _ = abi::sem_timedwait(self.inner, 0);
+        loop {
+            let mut guard = self.inner.lock();
+            if guard.locked == false {
+                guard.locked = true;
+                return;
+            } else {
+                let prio = abi::get_priority();
+                let id = abi::getpid();
+
+                guard.blocked_task.push(prio, id);
+                abi::block_current_task();
+                drop(guard);
+                abi::yield_now();
+            }
+        }
     }
 
     #[inline]
     pub unsafe fn unlock(&self) {
-        let _ = abi::sem_post(self.inner);
+        let mut guard = self.inner.lock();
+        guard.locked = false;
+        if let Some(tid) = guard.blocked_task.pop() {
+            abi::wakeup_task(tid);
+        }
     }
 
     #[inline]
     pub unsafe fn try_lock(&self) -> bool {
-        let result = abi::sem_trywait(self.inner);
-        result == 0
+        let mut guard = self.inner.lock();
+        if guard.locked == false {
+            guard.locked = true;
+        }
+        guard.locked
     }
 
     #[inline]
     pub unsafe fn destroy(&self) {
-        let _ = abi::sem_destroy(self.inner);
     }
 }