Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

[Darren Hart <dvhart@infradead.org>]

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@linutronix.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.

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.

-- 
Darren Hart
Intel Open Source Technology Center