Re: Is module refcounting racy?

From: Nick Piggin
Date: Tue Apr 06 2010 - 01:06:13 EST

On Thu, Apr 01, 2010 at 08:55:59AM -0700, Linus Torvalds wrote:
> On Thu, 1 Apr 2010, Nick Piggin wrote:
> >
> > I think it can be done racelessly with my patch, which is not really too
> > much overhead. I think if this is considered too much, then we should
> > either fix code and preferably de-export and remove module_refcount from
> > drivers, or remove module removal completely.
> I doubt your patch matters too much, but I like it conceptually and it
> seems to be a nice basis for perhaps doing something clever in the long
> run.
> [ ie avoiding the stop_machine and instead perhaps doing some optimistic
> thing like "see if we seem to be unused right now, then unregister us,
> and see - after unregistering - that the usage counts haven't increased,
> and re-register if they have. ]

That's true, reducing the requirement for stop_machine is always a nice
thing to have.

Also if anyone else is looking at a way to do _really_ scalable
refcounting elsewhere, this could be a good template (I certainly looked
here first when trying to get ideas for vfsmount refcounting).

> So I'd like to apply it as a "good improvement, even if module unloading
> which is the only thing that _should_ care deeply should already be under
> stop-machine".
> But I'd like an ack or two first.

Sure, I'll let Rusty push it to you when he's happy with it.

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