linux-kernel - Re: [RFC patch 4/7] futex: Add support for attached futexes

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Date:	Sun, 3 Apr 2016 11:59:07 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Darren Hart <darren@...art.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [RFC patch 4/7] futex: Add support for attached futexes

On Sat, 2 Apr 2016, Peter Zijlstra wrote:
> On Sat, Apr 02, 2016 at 11:09:18AM -0000, Thomas Gleixner wrote:
> > +	/*
> > +	 * Lock the global hash bucket. Decrement global state refcount. If 0
> > +	 * remove it from the global hash and free it.
> > +	 */
> > +	spin_lock(&hb->lock);
> > +	if (--fs->refcount == 0)
> > +		hb_remove_q(q, hb);
> > +	else
> > +		fs = NULL;
> > +	spin_unlock(&hb->lock);
> 
> So you could play funny games like:
> 
> 	if (atomic_add_unless(&fs->recount, -1, 1))
> 		return;
> 
> 	spin_lock(&hb->lock);
> 	if (atomic_dec_return(&fs->refcount) == 0)
> 		hb_remove_q(q, hb);
> 	else
> 		fs = NULL;
> 	spin_unlock(&hb->lock);
> 
> To avoid taking that lock entirely in the 'fast' path, but I'm not sure
> how performance critical this path is.

Attach/detach is not really critical. The futex ops are critical and they do
not touch the global hash bucket lock at all.

Thanks,

	tglx