Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

[Michael Kerrisk (man-pages)]

On 06/23/2016 08:28 PM, Darren Hart wrote:
> On Thu, Jun 23, 2016 at 07:26:52PM +0200, Thomas Gleixner wrote:
> > On Thu, 23 Jun 2016, Darren Hart wrote:
> > > On Thu, Jun 23, 2016 at 03:40:36PM +0200, Thomas Gleixner wrote:
> > > In my opinion, we should treat the timeout value as relative for FUTEX_WAIT
> > > regardless of the CLOCK used.
> >
> > Which requires even more changes as you have to select which clock you are
> > using for adding the base time.
>
> Right, something like the following?
>
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 33664f7..c39d807 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -3230,8 +3230,12 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
>  			return -EINVAL;
>
>  		t = timespec_to_ktime(ts);
> -		if (cmd == FUTEX_WAIT)
> -			t = ktime_add_safe(ktime_get(), t);
> +		if (cmd == FUTEX_WAIT) {
> +			if (cmd & FUTEX_CLOCK_REALTIME)
> +				t = ktime_add_safe(ktime_get_real(), t);
> +			else
> +				t = ktime_add_safe(ktime_get(), t);
> +		}
>  		tp = &t;
>  	}
>  	/*

Just in the interests of readability/maintainability, might it not
make some sense to recode the timeout handling for FUTEX_WAIT
within futex_wait(). I think that part of the reason we're in this
mess of inconsistency is that timeout interpretation is being handled
at too many different points in the code.

> And as a follow-on, what is the reason for FUTEX_LOCK_PI only using
> CLOCK_REALTIME? It seems reasonable to me that a user may want to wait a
> specific amount of time, regardless of wall time.

Yes, that's another weird inconsistency.

Thanks,

Michael


--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/