From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dario Faggioli <dario.faggioli@citrix.com>
Subject: Re: [PATCH 06/19] xen: credit2: read NOW() with the
 proper runq lock held
Date: Thu, 7 Jul 2016 12:28:59 +0200
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To: George Dunlap <george.dunlap@citrix.com>, Jan Beulich <JBeulich@suse.com>
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On Wed, 2016-07-06 at 17:10 +0100, George Dunlap wrote:
> On Mon, Jun 20, 2016 at 8:56 AM, Jan Beulich <JBeulich@suse.com>
> wrote:
> >=C2=A0
> > > > > On 18.06.16 at 01:12, <dario.faggioli@citrix.com> wrote:
> > > Yet another situation very similar to 779511f4bf5ae
> > > ("sched: avoid races on time values read from NOW()").
> > >=20
> > > In fact, when more than one runqueue is involved, we need
> > > to make sure that the following does not happen:
> > > =C2=A01. take the lock of 1st runq
> > > =C2=A02. now =3D NOW()
> > > =C2=A03. take the lock of 2nd runq
> > > =C2=A04. use now
> > >=20
> > > as, if we have to wait at step 3, the value in now may
> > > be stale when we get to use it at step 4.
> > Is this really meaningful here? We're talking of trylocks, which
> > don't
> > incur any delay other than the latency of the LOCKed (on x86)
> > instruction to determine lock availability.
> This makes sense to me -- Dario?
>=20
Yes, I think this patch is, after all, not really necessary.

Thanks and Regards,
Dario
--=20
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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