netdev - Re: [PATCH 7/9] rhashtable: Per bucket locks & deferred expansion/shrinking

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Message-ID: <20150116220735.GA12614@acer.localdomain>
Date:	Fri, 16 Jan 2015 22:07:36 +0000
From:	Patrick McHardy <kaber@...sh.net>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	David Laight <David.Laight@...LAB.COM>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"john.r.fastabend@...el.com" <john.r.fastabend@...el.com>,
	"josh@...htriplett.org" <josh@...htriplett.org>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH 7/9] rhashtable: Per bucket locks & deferred
 expansion/shrinking

On 16.01, Thomas Graf wrote:
> On 01/16/15 at 07:35pm, Patrick McHardy wrote:
> > On 16.01, Thomas Graf wrote:
> > > Right,but that's a Netlink dump issue and not specific to rhashtable.
> > 
> > Well, rhashtable (or generally resizing) will make it a lot worse.
> > Usually we at worst miss entries which were added during the dump,
> > which is made up by the notifications.
> > 
> > With resizing we might miss anything, its completely undeterministic.
> > 
> > > Putting the sequence number check in place should be sufficient
> > > for sets, right?
> > 
> > I don't see how. The problem is that the ordering of the hash changes
> > and it will skip different entries than those that have already been
> > dumped.
> 
> Since an resize can only be triggered through insert/remove you
> simply bump a sequence number when you insert/remove and have
> userspace restart the dump when NLM_F_DUMP_INTR is set.

I'm afraid that's not good enough. The resize operation is deferred,
so even if userspace does not perform an operation after starting
the dump, the hash might change.

We can obviously work around that by incrementing a generation
counter in rhashtable. The main problem I see is that with something
very actively changing the ruleset, we might never complete a dump.

Dumps are usually rare, I think its preferrable to defer rehashing.
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