linux-kernel - Re: [PATCH 2/4] futex2: Implement vectorized wait

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Message-ID: <20210818162034.GA26408@worktop.programming.kicks-ass.net>
Date:   Wed, 18 Aug 2021 18:20:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     André Almeida <andrealmeid@...labora.com>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kernel@...labora.com, krisman@...labora.com,
        linux-api@...r.kernel.org, libc-alpha@...rceware.org,
        mtk.manpages@...il.com, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 2/4] futex2: Implement vectorized wait

On Wed, Aug 18, 2021 at 01:00:57PM +0200, Thomas Gleixner wrote:
> > +/**
> > + * struct futex_waitv - A waiter for vectorized wait
> > + * @val:   Expected value at uaddr
> > + * @uaddr: User address to wait on
> > + * @flags: Flags for this waiter
> > + */
> > +struct futex_waitv {
> > +	__u64 val;
> 
> Again. Why u64?

So I think the idea was that if we're going to do new syscalls, we
should cater for future extentions, one of which was 64bit futexes (for
64bit archs) (along with u{8,16,32})

The previous set of patches implemented a more complete replacement ABI
-- which I rather liked, however the implementation was completely
disjoint from the existing futexes, which was a non-starter for me.

Anyway, yes, current futexes are u32, but if we want to ever do u64
futexes, we should either do this syscall with a u64, or already plan to
retire the whole syscall.

Obiously this would've made good Changelog material, but alas it wasn't
there.