linux-kernel - Re: [PATCH] locking: Remove rt_rwlock_is

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Date:   Fri, 10 Sep 2021 18:16:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] locking: Remove rt_rwlock_is_contended()

On Tue, Sep 07, 2021 at 12:34:58PM +0200, Sebastian Andrzej Siewior wrote:
> rt_rwlock_is_contended() has not users. It makes no sense to use it as
> rwlock_is_contended() because it is a sleeping lock on RT and preemption
> is possible. It reports always != 0 if used by a writer and even if
> there is a waiter then the lock might not be handed over if the
> current owner has the highest priority.

I'm confused now... so first you have two patches that wire up
{spin,rwlock}_is_contended() and how you're arguing we shouldn't do
that?

AFAICT the _is_contended() can still use useful even with preemption,
the typicla use case is a long lock-holder deciding to drop the lock in
order to let someone else in. That still works with preemptible locks,
no?