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

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Message-ID: <alpine.DEB.2.00.1105101531160.4023@router.home>
Date:	Tue, 10 May 2011 15:33:31 -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:

> Le mardi 10 mai 2011 à 14:38 -0500, Christoph Lameter a écrit :
>
> > Optimizing? You think about this as concurrency issue between multiple
> > cpus. That is fundamentally wrong. This is dealing with access to per cpu
> > data and the concurrency issues are only with code running on the *same*
> > cpu.
> >
>
> If you enable irqs, then this object can be allocated by _this_ cpu and
> given to another one.

That will cause an incrementing of the tid.

> Another cpu can free the page, forcing you to call a very expensive
> function, that might give obsolete result as soon it returns.

No the other cpu cannot free the page since the page is pinned by
the current cpu (see PageFrozen()).

> 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.