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

Peter Zijlstra
2023-02-23T12:32:54Z

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.