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

Mathieu Desnoyers
2008-11-07T16:56:25Z

* 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.

Mathieu

> > > /*
> > > * Caller must provide locking to protect *caller_state
> > > */
> >
> > NO! This is meant to be LOCK FREE!
>
> Absolutely.
>
> David

--
Mathieu Desnoyers
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