linux-kernel - Re: [PATCH] locking/rtmutex: remove unused cmpxchg

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Message-ID: <87r1zfxtne.fsf@nanos.tec.linutronix.de>
Date:   Fri, 31 Jan 2020 21:23:01 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Davidlohr Bueso <dave@...olabs.net>,
        Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/rtmutex: remove unused cmpxchg_relaxed

Davidlohr Bueso <dave@...olabs.net> writes:
> On Tue, 21 Jan 2020, Alex Shi wrote:

  Subject: locking/rtmutex: remove unused cmpxchg_relaxed

should be

  Subject: locking/rtmutex: Remove unused rt_mutex_cmpxchg_relaxed()

You're not removing cmpxchg_relaxed, right?

>> No one use this macro after it was introduced. Better to remove it?

Please make that factual.

 The macro was never used at all. Remove it.

> You also need to remove it for the CONFIG_DEBUG_RT_MUTEXES=y case.

Yes.

> Hmm unrelated, but do we want CCAS for rtmutex fastpath? Ie:
>
>      (l->owner == c && cmpxchg_acquire(&l->owner, c, n) == c)
>
> That would optimize for the contended case and avoid the cmpxchg - it would
> also help if we ever do the top-waiter spin thing.

Not sure if it buys much, but it kinda makes sense.

Thanks,

        tglx