linux-kernel - Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX

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Date:	Fri, 24 Jun 2016 10:13:49 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Darren Hart <dvhart@...radead.org>
Cc:	mtk.manpages@...il.com, Thomas Gleixner <tglx@...utronix.de>,
	Matthieu CASTET <matthieu.castet@...rot.com>,
	linux-kernel@...r.kernel.org, Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

On 06/23/2016 09:53 PM, Darren Hart wrote:
> On Thu, Jun 23, 2016 at 08:35:15PM +0200, Michael Kerrisk (man-pages) wrote:
>> Hi Darren,
>>
>> On 06/23/2016 06:16 PM, Darren Hart wrote:
>>> On Thu, Jun 23, 2016 at 03:40:36PM +0200, Thomas Gleixner wrote:
>>>> On Thu, 23 Jun 2016, Michael Kerrisk (man-pages) wrote:
>>>>> On 06/23/2016 09:18 AM, Thomas Gleixner wrote:
>>>>> Once upon a time, you told me the following:
>>>>>
>>>>> On 15 May 2014 at 16:14, Thomas Gleixner <tglx@...utronix.de> wrote:
>>>>>> On Thu, 15 May 2014, Michael Kerrisk (man-pages) wrote:
>>>>>>> And that universe would love to have your documentation of
>>>>>>> FUTEX_WAKE_BITSET and FUTEX_WAIT_BITSET ;-),
>>>>>>
>>>>>> I give you almost the full treatment, but I leave REQUEUE_PI to Darren
>>>>>> and FUTEX_WAKE_OP to Jakub. :)
>>>>>> [...]
>>>>>> FUTEX_CLOCK_REALTIME
>>>>>>
>>>>>>         This option bit can be ored on the futex ops FUTEX_WAIT_BITSET
>>>>>>         and FUTEX_WAIT_REQUEUE_PI
>>>>>>
>>>>>>         If set the kernel treats the user space supplied timeout as
>>>>>>         absolute time based on CLOCK_REALTIME.
>>>>>>
>>>>>>         If not set the kernel treats the user space supplied timeout
>>>>>>         as relative time.
>>>>> Unfortunately, I should have checked the code more carefully...
>>>>
>>>> Me too :)
>>>
>>> Seems to be going around...
>>>
>>>>
>>>>> Looking more carefully at the code, I see understand the situation
>>>>> is the following:
>>>>>
>>>>> FUTEX_LOCK_PI
>>>>> 	Always uses CLOCK_REALTIME
>>>>> 	'timeout' is absolute
>>>>
>>>> Yes.
>>>>
>>>>> FUTEX_WAIT_REQUEUE_PI
>>>>> 	Choice of clock (CLOCK_REALTIME vs CLOCK_MONOTONIC) is
>>>>>         	determined by presence or absence of
>>>>> 		FUTEX_CLOCK_REALTIME flag
>>>>> 	'timeout' is absolute
>>>>
>>>> Yes
>>>>
>>>>> FUTEX_WAIT_BITSET
>>>>> 	Choice of clock (CLOCK_REALTIME vs CLOCK_MONOTONIC) is
>>>>>         	determined by presence or absence of
>>>>> 		FUTEX_CLOCK_REALTIME flag
>>>>> 	'timeout' is absolute
>>>>
>>>> Yes
>>>>
>>>>> FUTEX_WAIT
>>>>> 	Choice of clock (CLOCK_REALTIME vs CLOCK_MONOTONIC) is
>>>>>         	determined by presence or absence of
>>>>> 		FUTEX_CLOCK_REALTIME flag
>>>>> 	'timeout' is relative
>>>>
>>>> Yes.
>>>>
>>>>> I've amended the man page to describe those details.
>>>
>>> OK, that confirms my question, timeout interpretation as relative or absolute is
>>> based on the op code, not the CLOCK flag.
>>>
>>>>>
>>>>>> The flag was explicitely added to allow FUTEX_WAIT to hand in absolute time.
>>>>>
>>>>> When you say that the "flag was added", which flag do you mean? Or, did you
>>>>> mean: "applying Matthieu's patch will allow FUTEX_WAIT to hand in absolute
>>>>> time".
>>>>
>>>> I didn't express myself clearly. When Darren added the support for
>>>> CLOCK_REALTIME to FUTEX_WAIT I think he wanted to add absolute timeout
>>>> support. Anything else does not make sense.
>>>
>>> I sent that patch because reading the new man page it struck me as strange that
>>> FUTEX_WAIT was restricted to CLOCK_MONOTONIC and the other op codes were not,
>>> especially since FUTEX_WAIT is a just FUTEX_WAIT_BITSET with the mask set to
>>> ALL.
>>>
>>> I didn't realize the impact to relative/absolute interpretation of the timeout
>>> value at the time.
>>>
>>> I think it was a mistake to introduce a change that made FUTEX_WAIT interpret
>>> the timeout differently based on the CLOCK flag,
>>
>> I'm missing something. Where does it do that? As far as I can tell FUTEX_WAIT
>> always interprets the clock as relative, regardless of presence/absence of
>> FUTEX_CLOCK_REALTIME? Am I missing something?
>
> No you're not. The code as it stands today is always relative, but it gets the
> base time from the wrong clock source in the case of FUTEX_CLOCK_REALTIME.

Ahh yes, I'd clicked to that, but forgot to say so.

> I was stating that I think it would be a mistake to add absolute timeout to
> FUTEX_WAIT based on the FUTEX_CLOCK_REALTIME flag, which is how Thomas describes
> above his interpretation of my earlier change.

Got it now. Thanks for the clarification, Darren.

Cheers

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/