try_upgrade

Author: ShrigmaDev

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

@@ -918,6 +918,24 @@ impl<'a, T: ?Sized> RwLockReadGuard<'a, T> {
             None => Err(orig),
         }
     }
+
+    #[unstable(feature = "rwlock_try_upgrade", issue = "138559")]
+    pub fn try_upgrade(orig: Self) -> Result<RwLockWriteGuard<'a, T>, RwLockReadGuard<'a, T>> {
+        let rwl = &RwLock {
+            data: UnsafeCell::new(orig.data.as_ptr().read()),
+            poison: poison::Flag::new(),
+            inner: *orig.inner_lock,
+        };
+
+        // don't call the destructor
+        forget(orig);
+
+        // SAFETY: We have ownership of the read guard, so it must be in read mode already
+        match unsafe { rwl.inner.try_upgrade() } {
+            true => Ok(RwLockWriteGuard::new(rwl).unwrap_or_else(PoisonError::into_inner)),
+            false => Err(RwLockReadGuard::new(rwl).unwrap_or_else(PoisonError::into_inner)),
+        }
+    }
 }
 
 impl<'a, T: ?Sized> MappedRwLockReadGuard<'a, T> {

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

@@ -33,6 +33,11 @@ fn is_write_locked(state: Primitive) -> bool {
     state & MASK == WRITE_LOCKED
 }
 
+#[inline]
+fn is_read_locked(state: Primitive) -> bool {
+    state & MASK == READ_LOCKED
+}
+
 #[inline]
 fn has_readers_waiting(state: Primitive) -> bool {
     state & READERS_WAITING != 0
@@ -205,6 +210,19 @@ impl RwLock {
         }
     }
 
+    /// # Safety
+    ///
+    /// The 'RwLock' must be read-locked in order to call this. Calling this function in a loop
+    /// in at least 2 threads will deadlock.
+    #[inline]
+    pub unsafe fn try_upgrade(&self) -> bool {
+        debg_assert!(
+            is_read_locked(self.state),
+            "RwLock must be read locked to call `try_upgrade`"
+        );
+        self.state.compare_exchange(READ_LOCKED, WRITE_LOCKED, Acquire, Relaxed).is_ok()
+    }
+
     #[cold]
     fn write_contended(&self) {
         let mut state = self.spin_write();