Messages in this thread | | | Subject | Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op | From | "Michael Kerrisk (man-pages)" <> | Date | Thu, 23 Jun 2016 20:35:15 +0200 |
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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?
> while that interpretation is > independent of the CLOCK flag for all other op codes. > > In my opinion, we should treat the timeout value as relative for FUTEX_WAIT > regardless of the CLOCK used.
I realize it's historical, but it is really weird that FUTEX_WAIT interprets time timeout (relative vs absolute) differently from all of the other operations. > That would require a change to the man page to eliminate the relative/absolute > language in the FUTEX_CLOCK_REALTIME definition and explicit definitions of the > interpretation for each op code (as Matthew explains above). > > Do we agree on that?
Yes.
The man page changes are already in Git. My earlier reply contained the commit ref: http://git.kernel.org/cgit/docs/man-pages/man-pages.git/commit/?id=8064bfa5369c6856f606004d02e48ab275e05bed
Cheers,
Michael
-- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
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