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

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Message-ID: <20150120145551.GH20315@casper.infradead.org>
Date:	Tue, 20 Jan 2015 14:55:51 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	davem@...emloft.net, herbert@...dor.apana.org.au,
	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 01/20/15 at 02:31pm, Patrick McHardy wrote:
> On 20.01, Thomas Graf wrote:
> > Lock out table resizes while dumping Netlink sockets to user space.
> > This keeps disruptions to a minimum for readers which don't handle
> > the NLM_F_DUMP_INTR flag.
> 
> This doesn't lock them out for the duration of the entire dump of
> course, so the benefit seems rather small. Still with this patch,
> they will need to handle NLM_F_DUMP_INTR or will get unpredictable
> behaviour, in which case I'd think it makes more sense to not even
> try this, all it does is hide parts of the brokenness.

If it would lock out the resize for the entire dump I would not have
done patches 1 and 2 ;-)

I does provide better behaviour if the whole dump fits into a single
buffer or if it fits into 2 buffers and we are already dumping into
the 2nd buffer when the resize occurs. Otherwise we will see resizes
and thus tons of duplicates even in those scenarios even if no insert
or removal occurs in parallel.

In the case of Netlink diag that should be typical case. Most systems
will not have 1000s of Netlink sockets in parallel.

> An alternative would be to set a flag in ht when a dump begins that
> indicates to skip resizing operations and on the end of the dump
> perform any resizing operations that might be necessary. Herbert
> disagrees though and he might be right.

I don't like the flag as it prevents resizes (and possibly rehashes
further down the road) for a long period of time. The hashtable
becomes attackable.
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