Subject: Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 22 May 2013 06:00:30 -0700

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