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

From: "André Almeida" <andrealmeid@collabora.com>
To: Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>,
	Darren Hart <dvhart@infradead.org>,
	linux-kernel@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	kernel@collabora.com, krisman@collabora.com,
	linux-api@vger.kernel.org, libc-alpha@sourceware.org,
	mtk.manpages@gmail.com, Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH 2/4] futex2: Implement vectorized wait
Date: Wed, 18 Aug 2021 13:34:00 -0300	[thread overview]
Message-ID: <42c6c37d-dd35-e797-9194-4619c1456f85@collabora.com> (raw)
In-Reply-To: <20210818162034.GA26408@worktop.programming.kicks-ass.net>

Às 13:20 de 18/08/21, Peter Zijlstra escreveu:
> 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.
> 

Ops, I forgot to add the reasoning behind the 64 futexes. The idea is
that futex users want to be able to properly do 64bit atomic operations
on top of futex values:

[0]
https://lore.kernel.org/lkml/CAFTs51XAr2b3DmcSM4=qeU5cNuh0mTxUbhG66U6bc63YYzkzYA@mail.gmail.com/

[1]
https://lore.kernel.org/lkml/20210603195924.361327-1-andrealmeid@collabora.com/T/#m37bfbbd6ac76c121941defd1daea774389552674

[2] https://lists.boost.org/Archives/boost/2021/05/251508.php