linux-kernel - Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX

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Date:	Thu, 23 Jun 2016 20:41:09 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Darren Hart <dvhart@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	mtk.manpages@...il.com,
	Matthieu CASTET <matthieu.castet@...rot.com>,
	linux-kernel@...r.kernel.org, Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

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/