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Date: Wed, 15 May 2013 05:19:48 -0400 (EDT)
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC PATCH 2/2] mem: prepare address_space
	listener rcu style
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To: liu ping fan <qemulist@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Anthony Liguori <anthony@codemonkey.ws>, "Michael S. Tsirkin" <mst@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

> > > rcu_read_lock();
> > > map = rcu_dereference(d->cur_map)
> > > if (unlikely(d->prev_map!=map) {
> > >     d->root = map->root;
> > >     d->pgtbl = map->pgtbl;
> > > }
> > > ......
> > > rcu_read_unlock();
> > >
> > > Then it can avoid ABA problem.
> >
> > I don't see the assignment of prev_map, which is where the ABA problem
> > arises.

If you do this:

> rcu_read_lock();
> map = rcu_derefenrence(d->cur_map)
> if (unlikely(d->prev_map!=map) {
>     d->root = map->root;
>     d->pgtbl = map->pgtbl;
>     d->prev_map = map;
> }
> ....
> rcu_read_unlock();

It can happen.  The problem is that prev_map can change _after_
the rcu_read_unlock.  It is the same as your previous solution.
Please just use the load-and-check idiom.  I'll post my RCU
implementation as soon as I finish the documentation.

Paolo