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Date: Wed, 22 May 2013 06:00:30 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Roman Gushchin <klamm@yandex-team.ru>,
        Dipankar Sarma <dipankar@in.ibm.com>, zhmurov@yandex-team.ru,
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        "David S. Miller" <davem@davemloft.net>,
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        James Morris <jmorris@namei.org>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Patrick McHardy <kaber@trash.net>
Subject: Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu
 macro
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On Wed, May 22, 2013 at 05:28:47AM -0700, Eric Dumazet wrote:
> On Wed, 2013-05-22 at 02:58 -0700, Paul E. McKenney wrote:
> 
> > Now that I am more awake...
> > 
> > The RCU list macros assume that the list header is either statically
> > allocated (in which case no ACCESS_ONCE() or whatever is needed) or
> > that the caller did whatever was necessary to protect the list header,
> > whether that be holding the right lock, using rcu_dereference() when
> > traversing the pointer to the list header, or whatever.
> 
> Not sure what you mean, we do hold rcu_read_lock() here.
> 
> But when we jump back to begin, we do not do
> "rcu_read_unlock()/rcu_read_lock()" pair.

Right, rcu_read_lock() is part of the protection, but rcu_dereference()
is the other part.

All that aside, I can't claim that I understand what problem the various
patches would solve.  ;-)

							Thanx, Paul