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

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Message-ID: <20150116163202.GJ30132@acer.localdomain>
Date:	Fri, 16 Jan 2015 16:32:03 +0000
From:	Patrick McHardy <kaber@...sh.net>
To:	Thomas Graf <tgraf@...g.ch>
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 16.01, Thomas Graf wrote:
> 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?

If we have to block, the dumper if of course preferred. Taking the
mutex should do fine I guess?

I suppose walking both tables without any races would be rather
complicated.
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