linux-kernel - Re: [PATCH for 5.9 1/3] futex: introduce FUTEX

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Message-ID: <20200723112757.GN5523@worktop.programming.kicks-ass.net>
Date:   Thu, 23 Jul 2020 13:27:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Peter Oskolkov <posk@...k.io>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Darren Hart <dvhart@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Paul Turner <pjt@...gle.com>,
        Ben Segall <bsegall@...gle.com>, Aaron Lu <aaron.lwe@...il.com>
Subject: Re: [PATCH for 5.9 1/3] futex: introduce FUTEX_SWAP operation

On Wed, Jul 22, 2020 at 04:45:36PM -0700, Peter Oskolkov wrote:
> This patchset is the first step to open-source this work. As explained
> in the linked pdf and video, SwitchTo API has three core operations: wait,
> resume, and swap (=switch). So this patchset adds a FUTEX_SWAP operation
> that, in addition to FUTEX_WAIT and FUTEX_WAKE, will provide a foundation
> on top of which user-space threading libraries can be built.

The PDF and video can go pound sand; you get to fully explain things
here.

What worries me is how FUTEX_SWAP would interact with the future
FUTEX_LOCK / FUTEX_UNLOCK. When we implement pthread_mutex with those,
there's very few WAIT/WAKE left.

Also, why would we commit to an ABI without ever having seen the rest?

On another note: wake_up_process_prefer_current_cpu() is a horrific
function name :/ That's half to a third of the line limit.