linux-kernel - Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

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Message-ID: <alpine.DEB.2.02.1311261436290.30673@ionos.tec.linutronix.de>
Date:	Tue, 26 Nov 2013 14:53:22 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Darren Hart <dvhart@...ux.intel.com>
cc:	Davidlohr Bueso <davidlohr@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>, jeffm@...e.com,
	"Norton, Scott J" <scott.norton@...com>, tom.vaden@...com,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Waiman Long <Waiman.Long@...com>,
	Jason Low <jason.low2@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

On Mon, 25 Nov 2013, Darren Hart wrote:
> On Mon, 2013-11-25 at 20:47 +0100, Thomas Gleixner wrote:
> > So now your code melts down to:
> > 
> >  write(hb->waiters)    |    write(uaddr)
> >  mb                    |    read(hb->waiters)
> >  read(uaddr)
> > 
> > I fear you simply managed to make the window small enough that your
> > testing was not longer able expose it.
> 
> Does seem to be the case.

Actually not. It's protected by an atomic_inc() between the
write(uaddr) and read(hb->waiters) on the waker side.

It took me a while to realize, that get_futex_key_refs() is providing
the protection inadvertently. But as I explained we cannot rely on
that for all architectures.

Thanks,

	tglx








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