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

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Message-ID: <20150121103959.GR3012@acer.localdomain>
Date:	Wed, 21 Jan 2015 10:40:00 +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 08:58pm, Herbert Xu wrote:
> > On Wed, Jan 21, 2015 at 09:49:28AM +0000, Thomas Graf wrote:
> > >
> > > An entry can move between different tables and thus chains need to be
> > > marked to identify what list a lookup ended up searching in. It's not
> > > the nulls marker itself that is needed, it's the bits in the last next
> > > pointer identifying the list that the nulls marker allows to be used
> > > which are essential.
> > 
> > Can you describe in more detail how it's going to be used? I don't
> > see how I could use the bit if you need it to indicate the end of
> > the list.
> 
> Another thought: Instead of storing that bit in the next pointer, we
> could require the user to store this bit, i.e. two new function
> pointers to rhashtable_params, set_walk_bit() and get_walk_bit(),
> which take the hashed object as argument and if a rhashtable user
> requires consistent dumps he can provide these functions to store
> the flag.

On the danger of repeating myself, every (converted) user requires
that we at least keep the existing semantics since it is exposed to
userspace. My opinion is that NLM_F_DUMP_INTR is fine if userspace
indicates support, but without that, we have to take care of that
in the kernel.

An automatic restart handles this well. Userspace always had to
expect duplicates.

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