netdev - Re: [PATCH 3/3] netlink: Lock out table resizes while dumping Netlink sockets

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Date:	Wed, 21 Jan 2015 09:37:22 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Patrick McHardy <kaber@...sh.net>, davem@...emloft.net,
	paulmck@...ux.vnet.ibm.com, ying.xue@...driver.com,
	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 3/3] netlink: Lock out table resizes while dumping
 Netlink sockets

On 01/21/15 at 04:08pm, Herbert Xu wrote:
> On Tue, Jan 20, 2015 at 03:35:56PM +0000, Thomas Graf wrote:
> > On 01/20/15 at 03:21pm, Patrick McHardy wrote:
> > > I think its preferrable to make the need to handle NETLINK_F_DUMP_INTR
> > > as noticable as possible and not hide it. Silent failure is the worst
> > > kind of failure.
> > 
> > I agree to that. The point here is to avoid unnecessary use of
> > NETLINK_F_DUMP_INTR if all entries fit into a single message buffer.
> 
> OK I think I have a solution for you guys.  But first you'll need to
> wait for me to undo the nulls stuff so I can steal that bit which
> is central to my solution.

Without having seen your code, can we make it configurable on what
the bit is used for? Use of nulls marker is a strict requirement for
some targeted users of rhashtable.

> Essentially I need a bit to indicate an entry in the bucket chain
> should be skipped, either because it has just been removed or that
> it is a walker entry (see xfrm_state_walk).
> 
> The way it'll work then is exactly the same as xfrm_state_walk,
> except that the linked list is broken up into individual buckets.
> 
> Of course we'll still need to postpone resizes (and rehashes which
> is what my work is about) during a walk but I think that's a fair
> price to pay.

If I understand this correctly we also need to block out parallel
walkers and we need to start taking bucket locks while walking to
modify the walker mark bit in peace.

> This also means handling insertion failures but I think that
> should be acceptable if we make it based on a configurable maximum
> chain length along with forced resize/rehash where possible.
> 
> Note that this can be made optional, i.e., if the user can afford
> memory to do their own walking (e.g., xfrm_state), then none of
> this needs to happen and it'll just work as it does now.  IOW if
> you don't use this special rhashtable walk function then you're
> not affected.

That sounds like the best option to me.
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