On Fri, 29 Jan 2016, Davidlohr Bueso wrote:
> On Wed, 27 Jan 2016, Hugh Dickins wrote:
>
> > > + *
> > > + * The RCU read lock is taken as the inode is finally freed
> > > + * under RCU. If the mapping still matches expectations then
> > > the
> > > + * mapping->host can be safely accessed as being a valid
> > > inode.
> > > + */
> > > + rcu_read_lock();
> > > + if (READ_ONCE(page->mapping) != mapping ||
> > > + !mapping->host) {
> >
> > If you're being as paranoid as all the WARN_ON_ONCEs hereabouts imply,
> > then it would be better to do the inode = READ_ONCE(mapping->host)
> > before checking !inode rather than !mapping->host.
>
> Ok, it also reads a bit nicer than the above, which was simply avoiding
> a load in the again case.
>
> rcu_read_lock();
> inode = READ_ONCE(mapping->host);
Just a quick unthinking from-the-hip response to that, something
for you to think over before sending v5: without looking back
at the code, it's not obvious to me that it's safe to read
mapping->host before we've confirmed under rcu_read_lock()
that page->mapping still matches mapping.
>
> if (!inode || READ_ONCE(page->mapping) != mapping)
> rcu_read_unlock();
> put_page(page);
>
> goto again;
> }
