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

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Message-ID: <20150121120727.GC12570@casper.infradead.org>
Date:	Wed, 21 Jan 2015 12:07:27 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>, 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 11:59am, Patrick McHardy wrote:
> On 21.01, Thomas Graf wrote:
> > On 01/21/15 at 10:40am, Patrick McHardy wrote:
> > > An automatic restart handles this well. Userspace always had to
> > > expect duplicates.
> > 
> > Maybe I don't understand the restart yet. How do you restart if the
> > dump was already started and the user has read part of the dump?
> 
> You use the mutex to prevent concurrent resizes and dumps, for dumps
> that have left the kernel you simply reset the iterator state after
> a resize operation.

I see, so we possibly see a lot of duplicates but we will never miss
any entries.

So in summary we do:

2) We allow rhashtable users to opt in to Herbert's consistent walker
   relying on the use of an additional bit.

2) If user supports NLM_F_DUMP_INTR we block out resizes during
   message construction and set NLM_F_DUMP_INTR if a resize/rehash
   interrupted us while the user read.

3) If (1) and (2) are not available we do (2) but restart the dump
   on interruption thus causing duplicates but never missing any
   entries.

I think this offers good flexibility for new users and compatibility
for existing users.
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