netdev - Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access

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Message-ID: <20131009234040.GB14055@jtriplet-mobl1>
Date:	Wed, 9 Oct 2013 16:40:40 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply
 rcu_access_pointer() to avoid sparse false positive

On Wed, Oct 09, 2013 at 04:17:55PM -0700, Eric Dumazet wrote:
> On Wed, 2013-10-09 at 15:56 -0700, Josh Triplett wrote:
> 
> > I'd be really hesitant to introduce that type of direct assignment to an
> > __rcu pointer without wrapping it in some appropriately named macro, or
> > at the very least adding a comment.
> 
> Well, there is no special magic here, in this specific case :
> 
> - deleting an item in an rcu list
> 
> Check list_del_rcu(), and you'll notice there is no _barrier_
> 
> Adding correct barriers is good, but please do not add them when not
> needed.
> 
> It makes code hard to understand.

I'm not arguing for the inclusion of an unnecessary barrier.  I'm
arguing for something more self-documenting than:

> ACCESS(*ptr) = value;

that.  Constructs like list_del_rcu are much clearer, and not
open-coded.  Open-coding synchronization code is almost always a Bad
Idea.

- Josh Triplett
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