netdev - Re: [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt

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Message-ID: <20080111083010.GA2183@ff.dom.local>
Date:	Fri, 11 Jan 2008 09:30:10 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	davem@...emloft.net, dipankar@...ibm.com, netdev@...r.kernel.org
Subject: Re: [NET] ROUTE: fix rcu_dereference() uses in /proc/net/rt_cache

On Fri, Jan 11, 2008 at 11:00:20AM +1100, Herbert Xu wrote:
> On Fri, Jan 11, 2008 at 12:10:42AM +0100, Jarek Poplawski wrote:
> >
> > It seems this optimization could've a side effect: if during such a
> > loop updates are done, and r is seen !NULL during while() check, but
> > NULL after rcu_dereference(), the listing/counting could stop too
> > soon. So, IMHO, probably the first version of this patch is more
> > reliable. (Or alternatively additional check is needed before return.)
> 
> No, while the value of r->u.dst.rt_next can change between two readings,
> the value of r cannot.

...Then, of course, it's O.K.!

It looks like I'm really too lazy and/or these selfdocumenting features
of RCU are a bit overrated: one can never be sure which pointer is
really RCU protected without checking a few places?! So, after looking
at this rt_cache_get_next() and this patch only, it's looks like the
third candidate after seq->private and rtable...

Thanks for explanation and sorry for disturbing!
Jarek P.
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