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 <t...@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
