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

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Date:	Tue, 20 Jan 2015 15:05:46 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	'Patrick McHardy' <kaber@...sh.net>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"ying.xue@...driver.com" <ying.xue@...driver.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"netfilter-devel@...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 03:00pm, David Laight wrote:
> From: Patrick McHardy
> > 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.
> 
> Can you rely on being told when the dump completes?
> If the program is killed in the middle then it can't tell you.
> 
> I suspect you'd have to suppress resize for some time interval
> after a partial dump.
> Unfortunately two continuous dumps would be likely to suppress
> it forever. Maybe sleep user space dump requests for the first
> block while any resize (esp. grow) is pending.
> 
> What is passed to userspace as the 'continue from here' marker?
> Even without resize there are likely to be issues if something
> nearer the head of a hash chain being processed is deleted.

I assumed that the proposal would include a timer in the Netlink
cb which fires after 2s or so and cancels the dump. 2s is still
a long time and a lot of inserts can happen in the meantime.

This may become more viable once insert()s can fail above a
certain watermark as Herbert suggests.
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