linux-kernel - Re: [PATCH v5] futex: Remove requirement for lock_page in get_futex

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Message-ID: <alpine.DEB.2.11.1602041845280.25254@nanos>
Date:	Thu, 4 Feb 2016 18:50:07 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Mel Gorman <mgorman@...hsingularity.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Chris Mason <clm@...com>, Darren Hart <dvhart@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dbueso@...e.de>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v5] futex: Remove requirement for lock_page in
 get_futex_key

On Thu, 4 Feb 2016, Davidlohr Bueso wrote:

> On Thu, 04 Feb 2016, Thomas Gleixner wrote:
> 
> > On Wed, 3 Feb 2016, Davidlohr Bueso wrote:
> > > +		 * We are not calling into get_futex_key_refs() in file-backed
> > > +		 * cases, therefore a successful atomic_inc return below will
> > > +		 * guarantee that get_futex_key() will continue to imply MB
> > > (B).
> > 
> > Can you please make that "MB (B)" part a bit more outstanding. I really had
> > to
> > search for it.
> 
> Hmm as you know this is mostly explained at the begining of the file, and we
> sprinkle MB (B) around the code based on that description. So I'm a bit
> confused
> as to why you don't like like that comment.

The other "MB (B)" places are more outstanding. It did not spring in my eye
immideately. So it's a pure cosmetic issue.

> > > +		 */
> > > +		if (WARN_ON_ONCE(!atomic_inc_not_zero(&inode->i_count))) {
> > > +			rcu_read_unlock();
> > > +			put_page(page);
> > > +
> > > +			goto again;
> > > +		}
> > 
> > Don't we need
> > 
> > 		smp_mb__after_atomic();
> > 
> > here to make it a full barrier on all architectures?
> 
> I had this initially but, as Peter mentioned, we get that barrier with the
> successful atomic_inc_not_zero call anyway. Or is it something else you had
> in mind?

Oh. I missed that comment from Peter. And yes, I missed that
atomic_inc_not_zero() already has the barrier in the success case. :(

Thanks,

	tglx