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

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Message-ID: <20131011002044.GA32546@jtriplet-mobl1>
Date:	Thu, 10 Oct 2013 17:20:44 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Eric Dumazet <eric.dumazet@...il.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 05:28:33PM -0700, Paul E. McKenney wrote:
> On Wed, Oct 09, 2013 at 05:12:40PM -0700, Eric Dumazet wrote:
> > On Wed, 2013-10-09 at 16:40 -0700, Josh Triplett wrote:
> > 
> > > that.  Constructs like list_del_rcu are much clearer, and not
> > > open-coded.  Open-coding synchronization code is almost always a Bad
> > > Idea.
> > 
> > OK, so you think there is synchronization code.
> > 
> > I will shut up then, no need to waste time.
> 
> As you said earlier, we should at least get rid of the memory barrier
> as long as we are changing the code.
> 
> Josh, what would you suggest as the best way to avoid the memory barrier,
> keep sparse happy, and not be too ugly?

The more I think about it, the more I realize that assigning an __rcu
pointer to an __rcu pointer *without* a memory barrier is a sufficiently
uncommon case that you probably *should* just write an open-coded
assignment.  Just please put a very clear comment right before it.

I'd originally thought it might make sense to have a macro similar to
rcu_assign_pointer, but I just don't think this is a common enough case,
and we don't want people thinking they can use this in general for __rcu
to __rcu assignments (most of which still need a memory barrier).

- Josh Triplett
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