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Date:	Thu, 10 Oct 2013 04:04:22 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Josh Triplett <josh@...htriplett.org>,
	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.

Interesting thread!

Sorry to chime in and asking a question:

Why do we need an ACCESS_ONCE here if rcu_assign_pointer can do without one?
In other words I wonder why rcu_assign_pointer is not a static inline function
to use the sequence point in argument evaluation (if I remember correctly this
also holds for inline functions) to not allow something like this:

E.g. we want to publish which lock to take first to prevent an ABBA problem
(extreme example):

rcu_assign_pointer(lockptr, min(lptr1, lptr2));

Couldn't a compiler spill the lockptr memory location as a temporary buffer
if the compiler is under register pressure? (yes, this seems unlikely if we
flushed out most registers to memory because of the barrier, but still... ;) )

This seems to be also the case if we publish a multi-dereferencing pointers
e.g. ptr->ptr->ptr.

Thanks,

  Hannes

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