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

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Message-ID: <20150116161530.GC15052@casper.infradead.org>
Date:	Fri, 16 Jan 2015 16:15:30 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	kernel@...r.kernel.org, herbert@...dor.apana.org.au,
	paulmck@...ux.vnet.ibm.com, edumazet@...gle.com,
	john.r.fastabend@...el.com, josh@...htriplett.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 7/9] rhashtable: Per bucket locks & deferred
 expansion/shrinking

On 01/16/15 at 04:03pm, Patrick McHardy wrote:
> On 16.01, Thomas Graf wrote:
> > A walker may not see insertions that occur after the walker was started
> > if resizing is enabled. Is that a problem for nftables?
> 
> No, that would be Ok. The case I'm wondering about is:
> 
> - insertion
> - resize starts
> - another insertion
> - walker, resize not finished yet

Correct, walker may not see "another insertion". The window for this
behavior to occur is not the full resize operation, just the linking
period, but it may occur. The length of the window is typically
equal to a grace period.

We can provide a synchronization function to block the dumper or the
insertion/removal until the linking is complete. The latter would
give the old runtime behaviour back (variable runtime of insert),
the blocked dumper might be preferred. What do you think?
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