linux-kernel - Re: Is module refcounting racy?

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Message-ID: <20100406050549.GA11191@laptop>
Date:	Tue, 6 Apr 2010 15:05:49 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Nick Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org,
	Jon Masters <jonathan@...masters.org>
Subject: Re: Is module refcounting racy?

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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