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

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Date:   Tue, 8 Jun 2021 14:03:50 +0300
From:   Andrey Semashev <andrey.semashev@...il.com>
To:     Nicholas Piggin <npiggin@...il.com>,
        André Almeida <andrealmeid@...labora.com>
Cc:     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 4:25 AM, Nicholas Piggin wrote:
> 
> Are shared pthread mutexes using existing pthread APIs that are today
> implemented okay with futex1 system call a good reason to constrain
> futex2 I wonder? Or do we have an opportunity to make a bigger change
> to the API so it suffers less from non deterministic latency (for
> example)?

If futex2 is not able to cover futex1 use cases then it cannot be viewed 
as a replacement. In the long term this means futex1 cannot be 
deprecated and has to be maintained. My impression was that futex1 was 
basically unmaintainable(*) and futex2 was an evolution of futex1 so 
that users of futex1 could migrate relatively easily and futex1 
eventually removed. Maybe my impression was wrong, but I would like to 
see futex2 as a replacement and extension of futex1, so the latter can 
be deprecated at some point.

In any case, creating a new API should consider requirements of its 
potential users. If futex2 is intended to eventually replace futex1 then 
all current futex1 users are potential users of futex2. If not, then the 
futex2 submission should list its intended users, at least in general 
terms, and their requirements that led to the proposed API design.

(*) I use "unmaintainable" in a broad sense here. It exists and works in 
newer kernel versions and may receive code changes that are necessary to 
keep it working, but maintainers refuse any extensions or modifications 
of the code, mostly because of its complexity.