Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

[Thomas Gleixner]

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@xxxxxxxxxxxxx> 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 :)
 
> 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.
> 
> > 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.

Thanks,

	tglx