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

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Date:	Thu, 23 Jun 2016 11:28:21 -0700
From:	Darren Hart <dvhart@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Michael Kerrisk (man-pages)" <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 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;
 	}
 	/*

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.

-- 
Darren Hart
Intel Open Source Technology Center