Panic from debug assertion when calling `RwLockWriteGuard::downgrade_to_upgradeable`

[an-owl]

RwLock::try_upgradeable_read() is

    pub fn try_upgradeable_read(&self) -> Option<RwLockUpgradableGuard<T, R>> {
        if self.lock.fetch_or(UPGRADED, Ordering::Acquire) & (WRITER | UPGRADED) == 0 {
            Some(RwLockUpgradableGuard {
                phantom: PhantomData,
                inner: self,
                data: unsafe { &*self.data.get() },
            })
        } else {
            // We can't unflip the UPGRADED bit back just yet as there is another upgradeable or write lock.
            // When they unlock, they will clear the bit.
            None
        }
    }

RwLockUpgradableGuard::try_upgrade_internal() contains compare_exchange( &inner.lock, UPGRADED, WRITER, Ordering::Acquire, Ordering::Relaxed,strong ) which clears UPGRADED and sets WRITER.

If try_upgrade_internal() is called then UPGRADED is cleared and WRITER is set. If try_upgradeable_read() is called on another thread this will set UPGRADED and return None, as it mentions in the fail branch it cant clear UPGRADED and the owner of the lock must clear it. However a number of debug assertions particularly in RwLockWriteGuard::downgrade_to_upgradeable and RwLockUpgradableGuard::drop() will panic in this event, even though RwLockWriteGuard::downgrade_to_upgradeable clears both UPGRADED and WRITER

[zesterer]

Thanks for the report, I'll try to look over this in more detail soon.