linux-kernel - Re: Is module refcounting racy?

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Message-Id: <201004061209.10647.rusty@rustcorp.com.au>
Date:	Tue, 6 Apr 2010 12:09:10 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>, Nick Piggin <npiggin@...il.com>,
	linux-kernel@...r.kernel.org, Jon Masters <jonathan@...masters.org>
Subject: Re: Is module refcounting racy?

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
caller.

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.

Yep.

Acked-by: Rusty Russell <rusty@...tcorp.com.au>

Cheers,
Rusty.
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