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From: Zebediah Figura <zfigura@codeweavers.com>
To: Andrey Semashev <andrey.semashev@gmail.com>,
	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>
Subject: Re: [PATCH v4 00/15] Add futex2 syscalls
Date: Tue, 8 Jun 2021 12:06:58 -0500	[thread overview]
Message-ID: <4c16b622-d2a8-c9f7-4c0b-80c9dbd8ef2c@codeweavers.com> (raw)
In-Reply-To: <3fca0afa-d9db-a176-aad1-ff7db21ba4a2@gmail.com>

On 6/8/21 8:18 AM, 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.
> 
> The project you mention is probably Wine and its derivatives. Those need 
> variable sized futexes and "wait for multiple" operation. I'm not sure 
> about the "no plan to merge it back" part, I probably missed it in an 
> earlier discussion. There are multiple different patches and versions 
> out there, and I don't know which one it refers to. But WaitOnAddress 
> and WaitForMultipleObjects APIs are very important and I would assume 
> Wine wants to emulate those with best efficiency.

See [0]. The short version is that we can't use futexes the way that 
out-of-tree patch set does, due to compatibility and robustness 
problems. I wrote said patch set and I'm currently working on a 
different solution for upstreaming.

We also can't exactly use futexes to back WaitOnAddress() directly. We 
actually do currently, but for various complex reasons that needs to 
change, and none of the proposals for futex2 help the new implementation.

ἔρρωσο,
Zebediah

[0] 
https://lore.kernel.org/lkml/dab34fd2-b494-8686-bcd7-68beeba4f386@gmail.com/

  parent reply	other threads:[~2021-06-08 17:24 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-03 19:59 André Almeida
2021-06-03 19:59 ` [PATCH v4 01/15] futex2: Implement wait and wake functions André Almeida
2021-06-03 19:59 ` [PATCH v4 02/15] futex2: Add support for shared futexes André Almeida
2021-06-03 19:59 ` [PATCH v4 03/15] futex2: Implement vectorized wait André Almeida
2021-06-03 19:59 ` [PATCH v4 04/15] futex2: Implement requeue operation André Almeida
2021-06-03 19:59 ` [PATCH v4 05/15] futex2: Implement support for different futex sizes André Almeida
2021-06-04  0:23   ` kernel test robot
2021-06-06 19:12   ` Davidlohr Bueso
2021-06-06 23:01     ` Andrey Semashev
2021-06-03 19:59 ` [PATCH v4 06/15] futex2: Add compatibility entry point for x86_x32 ABI André Almeida
2021-06-03 19:59 ` [PATCH v4 07/15] docs: locking: futex2: Add documentation André Almeida
2021-06-06 19:23   ` Davidlohr Bueso
2021-06-03 19:59 ` [PATCH v4 08/15] selftests: futex2: Add wake/wait test André Almeida
2021-06-03 19:59 ` [PATCH v4 09/15] selftests: futex2: Add timeout test André Almeida
2021-06-03 19:59 ` [PATCH v4 10/15] selftests: futex2: Add wouldblock test André Almeida
2021-06-03 19:59 ` [PATCH v4 11/15] selftests: futex2: Add waitv test André Almeida
2021-06-03 19:59 ` [PATCH v4 12/15] selftests: futex2: Add requeue test André Almeida
2021-06-03 19:59 ` [PATCH v4 13/15] selftests: futex2: Add futex sizes test André Almeida
2021-06-03 19:59 ` [PATCH v4 14/15] perf bench: Add futex2 benchmark tests André Almeida
2021-06-03 19:59 ` [PATCH v4 15/15] kernel: Enable waitpid() for futex2 André Almeida
2021-06-04  4:51 ` [PATCH v4 00/15] Add futex2 syscalls Zebediah Figura
2021-06-04 17:04   ` André Almeida
2021-06-04 11:36 ` Nicholas Piggin
2021-06-04 20:01   ` André Almeida
2021-06-05  1:09     ` Nicholas Piggin
2021-06-05  8:56       ` Andrey Semashev
2021-06-06 11:57         ` Nicholas Piggin
2021-06-06 13:15           ` Andrey Semashev
2021-06-08  1:25             ` Nicholas Piggin
2021-06-08 11:03               ` Andrey Semashev
2021-06-08 11:13                 ` Greg KH
2021-06-08 11:44                   ` Peter Zijlstra
2021-06-08 14:31                     ` Davidlohr Bueso
2021-06-08 12:06                   ` Andrey Semashev
2021-06-08 12:33                     ` Greg KH
2021-06-08 12:35                     ` Greg KH
2021-06-08 13:18                       ` Andrey Semashev
2021-06-08 13:27                         ` Greg KH
2021-06-08 13:41                           ` Andrey Semashev
2021-06-08 17:06                         ` Zebediah Figura [this message]
2021-06-08 14:14                   ` André Almeida
2021-06-07 15:40       ` André Almeida
2021-06-08  1:31         ` Nicholas Piggin
2021-06-08  2:33         ` Davidlohr Bueso
2021-06-08  4:45           ` Nicholas Piggin
2021-06-08 12:26         ` Sebastian Andrzej Siewior
2021-06-08 14:23           ` Peter Zijlstra
2021-06-08 14:57             ` Sebastian Andrzej Siewior
2021-06-08 15:04             ` André Almeida
2021-06-08 18:08             ` Adhemerval Zanella
2021-06-08 18:19               ` Florian Weimer
2021-06-08 18:22                 ` Adhemerval Zanella
2021-06-09 16:26             ` David Laight

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