Re: [PATCH v4] futex: Remove requirement for lock_page in get_futex_key

[Hugh Dickins <hughd@google.com>]

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;
>                }