futex not catastrophically failing yet

Author: connortsui20

library/std/src/sync/rwlock/tests.rs

@@ -501,3 +501,11 @@ fn panic_while_mapping_write_unlocked_poison() {
 
     drop(lock);
 }
+
+#[test]
+fn test_downgrade_basic() {
+    let r = RwLock::new(());
+
+    let write_guard = r.write().unwrap();
+    let _read_guard = RwLockWriteGuard::downgrade(write_guard);
+}

library/std/src/sys/sync/rwlock/futex.rs

@@ -134,7 +134,8 @@ impl RwLock {
             // Wait for the state to change.
             futex_wait(&self.state, state | READERS_WAITING, None);
 
-            // FIXME this protocol does not work
+            // FIXME make sure this works
+            // FIXME this can probably be more elegant
             state = self.state.load(Relaxed);
             if state & MASK < MAX_READERS && !has_readers_waiting(state) {
                 match self.state.compare_exchange_weak(state, state + READ_LOCKED, Acquire, Relaxed)
@@ -145,10 +146,10 @@ impl RwLock {
                         continue;
                     }
                 }
-            } else {
-                // Otherwise, spin again after waking up.
-                state = self.spin_read();
             }
+
+            // Otherwise, spin again after waking up.
+            state = self.spin_read();
         }
     }
 
@@ -177,7 +178,7 @@ impl RwLock {
         }
     }
 
-    // FIXME this does not work
+    // FIXME make sure this works
     #[inline]
     pub unsafe fn downgrade(&self) {
         // Removes all the write bits and adds a single read bit.