Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb()

Nicolas Pitre
2008-11-07T20:18:28Z

On Fri, 7 Nov 2008, Mathieu Desnoyers wrote:

> * David Howells (dhowells@redhat.com) wrote:
> > Nicolas Pitre <nico@cam.org> wrote:
> >
> > > > I mean, the darned thing is called from sched_clock(), which can be
> > > > concurrently called on separate CPUs and which can be called from
> > > > interrupt context (with an arbitrary nesting level!) while it was running
> > > > in process context.
> > >
> > > Yes! And this is so on *purpose*. Please take some time to read the
> > > comment that goes along with it, and if you're still not convinced then
> > > look for those explanation emails I've already posted.
> >
> > I agree with Nicolas on this. It's abominably clever, but I think he's right.
> >
> > The one place I remain unconvinced is over the issue of preemption of a process
> > that is in the middle of cnt32_to_63(), where if the preempted process is
> > asleep for long enough, I think it can wind time backwards when it resumes, but
> > that's not a problem for the one place I want to use it (sched_clock()) because
> > that is (almost) always called with preemption disabled in one way or another.
> >
> > The one place it isn't is a debugging case that I'm not too worried about.
> >
>
> I am also concerned about the non-preemption off case.
>
> Then I think the function should document that it must be called with
> preempt disabled.

I explained several times already why I disagree. Preemption is not a
problem unless you're preempted away for long enough, or IOW if your
counter is too fast.

And no, ^Z on a process doesn't create preemption. This is a signal that
gets acted upon far away from the middle of cnt32_to_63().

Nicolas