netdev - Re: [Bugme-new] [Bug 33502] New: Caught 64-bit read from uninitialized memory in __alloc

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Date:	Wed, 11 May 2011 05:12:23 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Vegard Nossum <vegardno@....uio.no>,
	Pekka Enberg <penberg@...helsinki.fi>,
	casteyde.christian@...e.fr,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev@...r.kernel.org, bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org
Subject: Re: [Bugme-new] [Bug 33502] New: Caught 64-bit read from
 uninitialized memory in __alloc_skb

Le mardi 10 mai 2011 à 16:22 -0500, Christoph Lameter a écrit :
> On Tue, 10 May 2011, Eric Dumazet wrote:
> 
> > > No the other cpu cannot free the page since the page is pinned by
> > > the current cpu (see PageFrozen()).
> > >
> >
> > What happens then ? Other cpu calls kfree() on last nonfreed object for
> > this slab, and yet the page stay frozen ? How this page is going to be
> > freed at all ?
> 
> Yes the page stays frozen. The freed objects are used to replenish the
> percpu free list when it becomes empty.
> 
> The page is going to be freed when a kmalloc() finds that the per cpu
> freelist is empty and that the freelist of the page is also empty. Then
> interrupts are disabled, the old page is unfrozen and a new
> page is acquired for allocation.
> 
> > > > Maybe I am just tired tonight, this seems very obvious, I must miss
> > > > something.
> > >
> > > Yeah you are way off thinking about cpu to cpu concurrency issues that do
> > > not apply here.
> >
> > I fail to understand how current cpu can assert page ownership, if IRQs
> > are enabled, this seems obvious it cannot.
> 
> The cpu sets a page flag called PageFrozen() and points a per cpu pointer
> to the page.
> 
> 

So, if I understand you, there is no problem at all and no patch even
needed ? I can start a stress test and you guarantee there wont be a
crash ?

Sorry, its 5h11 in the morning here ;)



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