Re: [PATCH v8 3/5] virtio_balloon: introduce migration primitives to balloon pages

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2012-08-21 at 17:40 +0300, Michael S. Tsirkin wrote:
> > +             spin_lock(&vb->pages_lock);
> > +             page = list_first_or_null_rcu(&vb->pages, struct page, lru);
> 
> Why is list_first_or_null_rcu called outside
> RCU critical section here?

It looks like vb->pages_lock is the exclusive (or modification)
counterpart to the rcu-read-lock in this particular case, so it should
be fine.

Although for that same reason, it seems superfluous to use the RCU list
method since we're exclusive with list manipulations anyway.

> > +             if (!page) {
> > +                     spin_unlock(&vb->pages_lock);
> > +                     break;
> > +             }
> > +             /*
> > +              * It is safe now to drop page->mapping and delete this page
> > +              * from balloon page list, since we are grabbing 'pages_lock'
> > +              * which prevents 'virtballoon_isolatepage()' from acting.
> > +              */
> > +             clear_balloon_mapping(page);
> > +             list_del_rcu(&page->lru);
> > +             spin_unlock(&vb->pages_lock);