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

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Date:   Mon, 24 Oct 2022 15:31:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, john.p.donnelly@...cle.com,
        Hillf Danton <hdanton@...a.com>,
        Mukesh Ojha <quic_mojha@...cinc.com>,
        Ting11 Wang 王婷 <wangting11@...omi.com>
Subject: Re: [PATCH v3 2/5] locking/rwsem: Limit # of null owner retries for
 handoff writer

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?