futex not catastrophically failing yet

Author: connortsui20

library/std/src/sync/rwlock.rs

@@ -4,7 +4,7 @@ mod tests;
 use crate::cell::UnsafeCell;
 use crate::fmt;
 use crate::marker::PhantomData;
-use crate::mem::ManuallyDrop;
+use crate::mem::{forget, ManuallyDrop};
 use crate::ops::{Deref, DerefMut};
 use crate::ptr::NonNull;
 use crate::sync::{poison, LockResult, TryLockError, TryLockResult};
@@ -969,13 +969,18 @@ impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
     /// MORE DOCS COMING SOON.
     #[unstable(feature = "rwlock_downgrade", issue = "128203")]
     pub fn downgrade(s: Self) -> RwLockReadGuard<'a, T> {
+        let lock = s.lock;
+
         // SAFETY: We take ownership of a write guard, so we must already have the `RwLock` in write
         // mode, satisfying the `downgrade` contract.
-        unsafe { s.lock.inner.downgrade() };
+        unsafe { lock.inner.downgrade() };
+
+        // We don't want to call the destructor since that calls `write_unlock`.
+        forget(s);
 
         // SAFETY: We have just successfully called `downgrade`, so we fulfill the safety contract.
         unsafe {
-            RwLockReadGuard::new(s.lock)
+            RwLockReadGuard::new(lock)
                 .expect("We had the exclusive lock so we can't have panicked while holding it")
         }
     }

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

@@ -496,3 +496,11 @@ 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

@@ -136,7 +136,8 @@ 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)
@@ -147,10 +148,10 @@ impl RwLock {
                         continue;
                     }
                 }
-            } else {
-                // Otherwise, spin again after waking up.
-                state = self.spin_read();
             }
+
+            // Otherwise, spin again after waking up.
+            state = self.spin_read();
         }
     }
 
@@ -179,7 +180,7 @@ 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.