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

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Date:	Tue, 10 May 2011 16:22:05 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Eric Dumazet <eric.dumazet@...il.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

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.


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