Re: [PATCH v2 2/3] locking/rwsem: Enable early rwsem writer lock handoff

[Peter Zijlstra]

On Thu, Feb 16, 2023 at 04:09:32PM -0500, Waiman Long wrote:
> @@ -432,19 +433,39 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
>  		 * Mark writer at the front of the queue for wakeup.
>  		 * Until the task is actually later awoken later by
>  		 * the caller, other writers are able to steal it.
> +		 *
> +		 * *Unless* HANDOFF is set, in which case only the
> +		 * first waiter is allowed to take it.
> +		 *
>  		 * Readers, on the other hand, will block as they
>  		 * will notice the queued writer.
>  		 */
>  		wake_q_add(wake_q, waiter->task);
>  		lockevent_inc(rwsem_wake_writer);
> +
> +		if ((count & RWSEM_LOCK_MASK) || !(count & RWSEM_FLAG_HANDOFF))
> +			return;
> +
> +		/*
> +		 * If the rwsem is free and handoff flag is set with wait_lock
> +		 * held, no other CPUs can take an active lock. We can do an
> +		 * early handoff.
> +		 */
> +		adjustment = RWSEM_WRITER_LOCKED - RWSEM_FLAG_HANDOFF;
> +		atomic_long_set(&sem->owner, (long)waiter->task);
> +		waiter->task = NULL;
> +		atomic_long_add(adjustment, &sem->count);
> +		rwsem_del_waiter(sem, waiter);
> +		lockevent_inc(rwsem_wlock_ehandoff);
>  	}

*sigh*... you can't even properly copy/paste from the reader side :/
This is broken, the moment you do waiter->task = NULL, waiter can
dissapear from under you and that rwsem_del_waiter() is a UaF.

Nor did you unify the reader and writer side and still have a giant
trainwreck on your hands..

Surely all that isn't too hard... let me post it.