linux-kernel - Re: [PATCH v4] futex: Remove requirement for lock_page in get_futex

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Date:	Fri, 29 Jan 2016 10:46:22 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Hugh Dickins <hughd@...gle.com>, Davidlohr Bueso <dbueso@...e.de>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Chris Mason <clm@...com>, Darren Hart <dvhart@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v4] futex: Remove requirement for lock_page in
 get_futex_key

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;
>                }