linux-kernel - Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry

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Message-ID: <20130522130030.GB3431@linux.vnet.ibm.com>
Date:	Wed, 22 May 2013 06:00:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Roman Gushchin <klamm@...dex-team.ru>,
	Dipankar Sarma <dipankar@...ibm.com>, zhmurov@...dex-team.ru,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"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>
Subject: Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu
 macro

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

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