netdev - Re: [RFC PATCH]: Dynamically sized routing cache hash table.

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Message-ID: <20070306092209.GB12365@wotan.suse.de>
Date:	Tue, 6 Mar 2007 10:22:09 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, dada1@...mosbay.com,
	robert.olsson@....uu.se, paulmck@...ibm.com
Subject: Re: [RFC PATCH]: Dynamically sized routing cache hash table.

On Tue, Mar 06, 2007 at 01:17:06AM -0800, David Miller wrote:
> From: Nick Piggin <npiggin@...e.de>
> Date: Tue, 6 Mar 2007 10:11:12 +0100
> 
> > @@ -1449,6 +1472,12 @@ static struct dst_entry *ipv4_negative_a
> >  					  "%u.%u.%u.%u/%02x dropped\n",
> >  				NIPQUAD(rt->rt_dst), rt->fl.fl4_tos);
> >  #endif
> > +			/* XXX:
> > +			 * What if rt does not exist in rt_hash, but is in
> > +			 * old_rt_hash? Don't we have to also check there?
> > +			 * Similar questions for a couple of other places that
> > +			 * look at rt_hash, but not old_rt_hash.
> > +			 */
> >  			rt_del(h, hash, rt);
> >  			ret = NULL;
> >  		}
> 
> For the cases like ip_rt_redirect() I made the decision that we'll
> just not add the complexity of having to look in the old_rt_hash
> table.
> 
> In these kinds of cases it's OK to miss events, they will just happen
> again.
> 
> It's one of the nice things about the routing cache, if you lose
> information it's OK because we'll just cook up a new entry from
> the persistent backing store that is the real routing tables.
> And for events like redirects, if we miss it, we'll just send the
> packet to the wrong next-hop again and receive another redirect
> message which we'll (hopefully) propagate to a routine cache
> entry.

Ah, that's a very neat trick. OK so with that question out of the
way, there _may_ just be a few other places where you're working
with an rt_hash table outside an rcu read critical section.

I tried to fix up a couple of obvious ones, and I've just put in
the BUG_ON assertions for you to verify the rest.

> Thanks for your feedback patch Nick, I'll process it tomorrow
> hopefully after getting some sleep.

No problem. Thanks.
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