Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op
From: Michael Kerrisk (man-pages)
Date: Thu Jun 23 2016 - 14:35:25 EST
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@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 :)
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/