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

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Message-ID: <20080111091140.GB2183@ff.dom.local>
Date:	Fri, 11 Jan 2008 10:11:40 +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 09:30:10AM +0100, Jarek Poplawski wrote:
> 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...

OOPS! ...it seems we are talking about the same, properly documented
(second) poiner yet...

So, IOW: strictly speaking you are right, r can't change here, but I
meant r vs. the returned value! Before the patch the returned value
couldn't be NULL unless all elements of the list were looped. After
this patch it seems possible...

Jarek P.
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