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

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Message-ID: <20150121052907.GY9719@linux.vnet.ibm.com>
Date:	Tue, 20 Jan 2015 21:29:07 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Pablo Neira Ayuso <pablo@...filter.org>,
	Patrick McHardy <kaber@...sh.net>, Thomas Graf <tgraf@...g.ch>,
	David Laight <David.Laight@...LAB.COM>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"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 Wed, Jan 21, 2015 at 04:23:33PM +1100, Herbert Xu wrote:
> On Mon, Jan 19, 2015 at 01:01:21AM -0800, Paul E. McKenney wrote:
> >
> > One unconventional way of handling this is to associate the scan with
> > a one-to-one resize operation.  This can be implemented to have the
> > effect of taking a snapshot of the table.
> 
> The problem is that in general (not for netfilter, but certainly
> other rhashtable users such as netlink) dumps/walks can be started
> by ordinary users and I don't think we can afford creating a
> snapshot for each one, or can we?

Well, you -could- batch them up, so that a single snapshot covered
several users, and once that set was done and memory reclaimed, a second
snapshot could cover any additional users that requested dumps/walks in
the meantime.  Or are users allowed to walk arbitrarily slowly through
the table?

							Thanx, Paul

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