netdev - Re: PROBLEM: skb

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Message-Id: <E1IfnRx-0003Vu-00@gondolin.me.apana.org.au>
Date:	Thu, 11 Oct 2007 10:06:57 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	sfa.linux@...il.com (Santiago Font Arquer)
Cc:	netdev@...r.kernel.org
Subject: Re: PROBLEM: skb_clone SMP race?

Santiago Font Arquer <sfa.linux@...il.com> wrote:
>
>   If an skb with fast clone available (first "if" true) has
> references in different CPUs (skb->users>1) (I do not find explicit
> checks for this to be impossible), if skb_clone is called
> simultaneously over that skb, both callers can get the same clone (the
> "fast" clone) and different problems follow: wrong "clone_skb->users"
> (1 as expected by the caller, but it should be, to be true, 2),
> fclone_ref set to 3 involving further problems, ...

Fast clones are only used by TCP where the original skb is
never given to the outside world.  This plus the fact that
a given TCP socket is single-threaded makes it safe.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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