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

From: Andrey Semashev <andrey.semashev@gmail.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: "Nicholas Piggin" <npiggin@gmail.com>,
	"André Almeida" <andrealmeid@collabora.com>,
	acme@kernel.org,
	"Sebastian Andrzej Siewior" <bigeasy@linutronix.de>,
	corbet@lwn.net, "Davidlohr Bueso" <dave@stgolabs.net>,
	"Darren Hart" <dvhart@infradead.org>,
	fweimer@redhat.com, joel@joelfernandes.org, kernel@collabora.com,
	krisman@collabora.com, libc-alpha@sourceware.org,
	linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, malteskarupke@fastmail.fm,
	"Ingo Molnar" <mingo@redhat.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	pgriffais@valvesoftware.com, "Peter Oskolkov" <posk@posk.io>,
	"Steven Rostedt" <rostedt@goodmis.org>,
	shuah@kernel.org, "Thomas Gleixner" <tglx@linutronix.de>,
	z.figura12@gmail.com
Subject: Re: [PATCH v4 00/15] Add futex2 syscalls
Date: Tue, 8 Jun 2021 16:41:07 +0300	[thread overview]
Message-ID: <edc30790-e29c-723c-50c8-61e258c6508b@gmail.com> (raw)
In-Reply-To: <YL9wROdz4y/pETA1@kroah.com>

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.