linux-kernel - Re: Linux 2.6.25-rc2

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Date:	Tue, 19 Feb 2008 15:08:52 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Torsten Kaiser <just.for.lkml@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>,
	Eric Dumazet <dada1@...mosbay.com>
Subject: Re: Linux 2.6.25-rc2

* Pekka Enberg (penberg@...helsinki.fi) wrote:
> Hi Mathieu,
> 
> On Feb 19, 2008 4:02 PM, Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> > - stat(c, ALLOC_FASTPATH); seems to be using a var++, therefore
> >   indicating it is not reentrant if IRQs are disabled. Since those are
> >   only stats, I guess it's ok, but still weird.
> 
> What is not re-entrant?
> 

incrementing the variable with a "++" when interrupts are not disabled.
It's not an atomic add and it's racy. The code within stat() does
exactly this.

> On Feb 19, 2008 4:02 PM, Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> > Since this shows mostly with network card drivers, I think the most
> > plausible cause would be an IRQ nesting over kmem_cache_alloc_node and
> > calling it.
> 
> Yes, this can happen. Are you saying it is not safe to be in the
> lockless path when an IRQ triggers?

It should be safe, but I think Eric pointed the correct problem in his
reply.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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