linux-kernel - Re: [PATCH v4 00/15] Add futex2 syscalls

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Date:   Tue, 8 Jun 2021 16:41:07 +0300
From:   Andrey Semashev <andrey.semashev@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        André Almeida <andrealmeid@...labora.com>,
        acme@...nel.org, Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        corbet@....net, Davidlohr Bueso <dave@...olabs.net>,
        Darren Hart <dvhart@...radead.org>, fweimer@...hat.com,
        joel@...lfernandes.org, kernel@...labora.com,
        krisman@...labora.com, libc-alpha@...rceware.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, malteskarupke@...tmail.fm,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        pgriffais@...vesoftware.com, Peter Oskolkov <posk@...k.io>,
        Steven Rostedt <rostedt@...dmis.org>, shuah@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>, z.figura12@...il.com
Subject: Re: [PATCH v4 00/15] Add futex2 syscalls

On 6/8/21 4:27 PM, Greg KH wrote:
> On Tue, Jun 08, 2021 at 04:18:42PM +0300, Andrey Semashev wrote:
>> On 6/8/21 3:35 PM, Greg KH wrote:
>>> On Tue, Jun 08, 2021 at 03:06:48PM +0300, Andrey Semashev wrote:
>>>> On 6/8/21 2:13 PM, Greg KH wrote:
>>>
>>>>> So what's keeping the futex2 code from doing all that futex1 does so
>>>>> that the futex1 code can be deleted internally?
>>>>
>>>> I think, André will answer this, but my guess is, as stated above, this is a
>>>> lot of work and time while the intermediate version is already useful.
>>>
>>> useful to who?  I still do not understand what users will be needing
>>> this.  All I can tell is a single userspace program wants to use it, and
>>> that is a fork from the real project it was based on and that the
>>> maintainers have no plan to merge it back.
>>>
>>> So who does need/want this?
>>
>> I mentioned C++ std::atomic and Boost.Atomic before. Those need variable
>> sized futexes.
> 
> And has anyone converted them to use this new api to see if it works
> well or not?
> 
> As was pointed out to me numerous times when I tried to propose
> readfile(), you need a real user that can show and prove it is needed
> before we can take new syscalls, especially complex beasts like this
> one.

André has mentioned that he tested the patch set with patched Wine and 
glibc.

I didn't patch Boost.Atomic or std::atomic, but it doesn't look to be 
problematic. The only difference it would make there is to enable 
futex2-based implementation for multiple atomic sizes and set up flags 
to indicate the futex size, instead of only enabling futex-based 
implementation for 32-bit atomics.