1 files changed, 101 insertions, 0 deletions
diff --git a/library/std/src/sys/unix/locks/futex_mutex.rs b/library/std/src/sys/unix/locks/futex_mutex.rs
new file mode 100644
index 00000000000..99ba86e5f99
--- /dev/null
+++ b/library/std/src/sys/unix/locks/futex_mutex.rs
@@ -0,0 +1,101 @@
+use crate::sync::atomic::{
+    AtomicU32,
+    Ordering::{Acquire, Relaxed, Release},
+};
+use crate::sys::futex::{futex_wait, futex_wake};
+
+pub type MovableMutex = Mutex;
+
+pub struct Mutex {
+    /// 0: unlocked
+    /// 1: locked, no other threads waiting
+    /// 2: locked, and other threads waiting (contended)
+    futex: AtomicU32,
+}
+
+impl Mutex {
+    #[inline]
+    pub const fn new() -> Self {
+        Self { futex: AtomicU32::new(0) }
+    }
+
+    #[inline]
+    pub unsafe fn init(&mut self) {}
+
+    #[inline]
+    pub unsafe fn try_lock(&self) -> bool {
+        self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_ok()
+    }
+
+    #[inline]
+    pub unsafe fn lock(&self) {
+        if self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_err() {
+            self.lock_contended();
+        }
+    }
+
+    #[cold]
+    fn lock_contended(&self) {
+        // Spin first to speed things up if the lock is released quickly.
+        let mut state = self.spin();
+
+        // If it's unlocked now, attempt to take the lock
+        // without marking it as contended.
+        if state == 0 {
+            match self.futex.compare_exchange(0, 1, Acquire, Relaxed) {
+                Ok(_) => return, // Locked!
+                Err(s) => state = s,
+            }
+        }
+
+        loop {
+            // Put the lock in contended state.
+            // We avoid an unnecessary write if it as already set to 2,
+            // to be friendlier for the caches.
+            if state != 2 && self.futex.swap(2, Acquire) == 0 {
+                // We changed it from 0 to 2, so we just succesfully locked it.
+                return;
+            }
+
+            // Wait for the futex to change state, assuming it is still 2.
+            futex_wait(&self.futex, 2, None);
+
+            // Spin again after waking up.
+            state = self.spin();
+        }
+    }
+
+    fn spin(&self) -> u32 {
+        let mut spin = 100;
+        loop {
+            // We only use `load` (and not `swap` or `compare_exchange`)
+            // while spinning, to be easier on the caches.
+            let state = self.futex.load(Relaxed);
+
+            // We stop spinning when the mutex is unlocked (0),
+            // but also when it's contended (2).
+            if state != 1 || spin == 0 {
+                return state;
+            }
+
+            crate::hint::spin_loop();
+            spin -= 1;
+        }
+    }
+
+    #[inline]
+    pub unsafe fn unlock(&self) {
+        if self.futex.swap(0, Release) == 2 {
+            // We only wake up one thread. When that thread locks the mutex, it
+            // will mark the mutex as contended (2) (see lock_contended above),
+            // which makes sure that any other waiting threads will also be
+            // woken up eventually.
+            self.wake();
+        }
+    }
+
+    #[cold]
+    fn wake(&self) {
+        futex_wake(&self.futex);
+    }
+}