1 files changed, 72 insertions, 0 deletions
diff --git a/library/std/src/sys/xous/locks/rwlock.rs b/library/std/src/sys/xous/locks/rwlock.rs
new file mode 100644
index 000000000..618da758a
--- /dev/null
+++ b/library/std/src/sys/xous/locks/rwlock.rs
@@ -0,0 +1,72 @@
+use crate::os::xous::ffi::do_yield;
+use crate::sync::atomic::{AtomicIsize, Ordering::SeqCst};
+
+pub struct RwLock {
+    /// The "mode" value indicates how many threads are waiting on this
+    /// Mutex. Possible values are:
+    ///    -1: The lock is locked for writing
+    ///     0: The lock is unlocked
+    ///   >=1: The lock is locked for reading
+    ///
+    /// This currently spins waiting for the lock to be freed. An
+    /// optimization would be to involve the ticktimer server to
+    /// coordinate unlocks.
+    mode: AtomicIsize,
+}
+
+unsafe impl Send for RwLock {}
+unsafe impl Sync for RwLock {}
+
+impl RwLock {
+    #[inline]
+    #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
+    pub const fn new() -> RwLock {
+        RwLock { mode: AtomicIsize::new(0) }
+    }
+
+    #[inline]
+    pub unsafe fn read(&self) {
+        while !unsafe { self.try_read() } {
+            do_yield();
+        }
+    }
+
+    #[inline]
+    pub unsafe fn try_read(&self) -> bool {
+        // Non-atomically determine the current value.
+        let current = self.mode.load(SeqCst);
+
+        // If it's currently locked for writing, then we cannot read.
+        if current < 0 {
+            return false;
+        }
+
+        // Attempt to lock. If the `current` value has changed, then this
+        // operation will fail and we will not obtain the lock even if we
+        // could potentially keep it.
+        let new = current + 1;
+        self.mode.compare_exchange(current, new, SeqCst, SeqCst).is_ok()
+    }
+
+    #[inline]
+    pub unsafe fn write(&self) {
+        while !unsafe { self.try_write() } {
+            do_yield();
+        }
+    }
+
+    #[inline]
+    pub unsafe fn try_write(&self) -> bool {
+        self.mode.compare_exchange(0, -1, SeqCst, SeqCst).is_ok()
+    }
+
+    #[inline]
+    pub unsafe fn read_unlock(&self) {
+        self.mode.fetch_sub(1, SeqCst);
+    }
+
+    #[inline]
+    pub unsafe fn write_unlock(&self) {
+        assert_eq!(self.mode.compare_exchange(-1, 0, SeqCst, SeqCst), Ok(-1));
+    }
+}