linux-kernel - Re: Is module refcounting racy?

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Message-ID: <20100406073818.GE11191@laptop>
Date:	Tue, 6 Apr 2010 17:38:18 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Tue, Apr 06, 2010 at 08:19:23AM +0200, Eric Dumazet wrote:
> Le mardi 06 avril 2010 à 15:05 +1000, Nick Piggin a écrit :
> 
> > 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).
> 
> Yes, nice trick Nick, I was thinking about it for network code :)
> 
> Acked-by: Eric Dumazet <eric.dumazet@...il.com>
> 
> I confess the smp_wmb() in module_put() bothered me a bit until I saw it
> was only a barrier() on X86 (if !CONFIG_X86_OOSTORE)

Yep. smp_wmb() and smp_rmb() are both noops on x86 (OOSTORE is some
really obscure thing that we don't need to worry about really). On
POWER6/7 CPUs, it uses lwsync which is fairly cheap as well.

I think refcounting in _general_ needs a smp_wmb() (or, to be more
precise, probably a release barrier) before decrements because you don't
want previous futzing with the object to leak into after a final
decrement may be observed by another CPU. So it might be hard to avoid
anyway.

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