linux-kernel - Re: netlink: Add netlink_bound helper and use it in netlink

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Message-ID: <20150926194554.GE3572@htj.duckdns.org>
Date:	Sat, 26 Sep 2015 15:45:54 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>, cwang@...pensource.com,
	tom@...bertland.com, kafai@...com, kernel-team@...com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	torvalds@...ux-foundation.org, jiri@...nulli.us,
	nicolas.dichtel@...nd.com, tgraf@...g.ch, sfeldma@...il.com
Subject: Re: netlink: Add netlink_bound helper and use it in netlink_getname

Hello, Herbert.

On Sun, Sep 27, 2015 at 03:41:10AM +0800, Herbert Xu wrote:
> Thread 1			Thread 2
> sendmsg				getsockname
> 	netlink_autobind		netlink_getname
> 
> Thread 2 should not have to do anything special to guarantee that
> getsockname does not return garbage.  It must either be the bound
> portid if the autobind completed in thread 1 and is visible or it
> should return zero.
> 
> As it stands thread 2 may see a portid belonging to somebody else
> if it catches the autobind in thread 1 trying different portids
> while roving.

If the fact that thread 1 finished autobind isn't visible to thread 2,
it's valid for getsockname to return zero.  No ordering between the
two operations is defined.  If the fact that thread 1 finished
autobind is visible to thread 2, ordering is defined and because
ordering is transitive, by that very ordering, the port number is
visible to thread 2 too as long as thread 1 does proper barriering.

Thanks.

-- 
tejun
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