Re: [PATCH v3 2/5] locking/rwsem: Limit # of null owner retries for handoff writer

[Peter Zijlstra]

On Mon, Oct 17, 2022 at 05:13:53PM -0400, Waiman Long wrote:
> Commit 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically
> spin on owner") assumes that when the owner field is changed to NULL,
> the lock will become free soon.  That assumption may not be correct
> especially if the handoff writer doing the spinning is a RT task which
> may preempt another task from completing its action of either freeing
> the rwsem or properly setting up owner.

I'm confused again -- rwsem_*_owner() has
lockdep_assert_preemption_disabled(). But more specifically; why can the
RT task preempt a lock-op like that?