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

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Date:	Wed, 21 Jan 2015 12:09:41 +0000
From:	Patrick McHardy <kaber@...sh.net>
To:	Thomas Graf <tgraf@...g.ch>
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 21.01, Thomas Graf wrote:
> 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.

Yep, sounds good to me.
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