Re: Is module refcounting racy?

From: Rusty Russell
Date: Mon Apr 05 2010 - 22:39:24 EST

On Fri, 2 Apr 2010 02:25:59 am 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. ]

I dislike that we can see spurious failure for some random try_module_get

But perhaps that's inherent in module removal: someone can miss out, and if
you care, don't try to remove modules.

And grepping for try_module_get() reveals a suspicious (growing) number of
try_module_get(THIS_MODULE) which is almost always wrong. If we're not
perfect, maybe we should aim for simple?

> 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.


Acked-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

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