linux-kernel - Re: netlink: Add barrier to netlink

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Message-ID: <20150925150113.GD4449@mtj.duckdns.org>
Date:	Fri, 25 Sep 2015 11:01:13 -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 barrier to netlink_connect for theoretical case

Hello, Herbert.

On Fri, Sep 25, 2015 at 09:43:27AM +0800, Herbert Xu wrote:
> Well had you said this in the first place I would've fixed it a
> long time ago.  There aren't any in-kernel users right now and
> even if there were they'd have to do a connect/bind/sendmsg on
> the same socket in two threads at the same time.  But let's close
> this theoretical hole:

I'm not even sure we guarantee memory barrier on kernel/user
crossings.  In practice, we probably have enough barriers (e.g. some
syscall traps imply barrier) but I can't think of a reason why we'd
guarantee the existence of barrier there.  As an extreme example,
imagine UML on an architecture with relaxed memory model.

Thanks.

-- 
tejun
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