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From: Thomas Gleixner <tglx@linutronix.de>
To: Darren Hart <dvhart@linux.intel.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Mike Galbraith <efault@gmx.de>,
	jeffm@suse.com, "Norton, Scott J" <scott.norton@hp.com>,
	tom.vaden@hp.com, "Chandramouleeswaran, Aswin" <aswin@hp.com>,
	Waiman Long <Waiman.Long@hp.com>, Jason Low <jason.low2@hp.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup
Date: Tue, 26 Nov 2013 14:53:22 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.02.1311261436290.30673@ionos.tec.linutronix.de> (raw)
In-Reply-To: <1385409830.12342.41.camel@dvhart-mobl4.amr.corp.intel.com>

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









  parent reply	other threads:[~2013-11-26 13:53 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-11-23  0:56 [PATCH 0/5] futex: Wakeup optimizations Davidlohr Bueso
2013-11-23  0:56 ` [PATCH 1/5] futex: Misc cleanups Davidlohr Bueso
2013-11-23  6:52   ` Darren Hart
2013-11-23  0:56 ` [PATCH 2/5] futex: Check for pi futex_q only once Davidlohr Bueso
2013-11-23  6:33   ` Darren Hart
2013-11-24  5:19     ` Davidlohr Bueso
2013-11-23  0:56 ` [PATCH 3/5] futex: Larger hash table Davidlohr Bueso
2013-11-23  6:52   ` Darren Hart
2013-11-23  0:56 ` [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup Davidlohr Bueso
2013-11-23  1:25   ` Linus Torvalds
2013-11-23  3:03     ` Jason Low
2013-11-23  3:19     ` Davidlohr Bueso
2013-11-23  7:23       ` Darren Hart
2013-11-23 13:16       ` Thomas Gleixner
2013-11-24  3:46         ` Linus Torvalds
2013-11-24  5:15           ` Davidlohr Bueso
2013-11-25 12:01             ` Thomas Gleixner
2013-11-25 16:23           ` Thomas Gleixner
2013-11-25 16:36             ` Peter Zijlstra
2013-11-25 17:32               ` Thomas Gleixner
2013-11-25 17:38                 ` Peter Zijlstra
2013-11-25 18:55                 ` Davidlohr Bueso
2013-11-25 19:52                   ` Thomas Gleixner
2013-11-25 19:47         ` Thomas Gleixner
2013-11-25 20:03           ` Darren Hart
2013-11-25 20:26             ` Thomas Gleixner
2013-11-26 13:53             ` Thomas Gleixner [this message]
2013-11-23  4:05     ` Waiman Long
2013-11-23  5:40   ` Darren Hart
2013-11-23  5:42     ` Hart, Darren
2013-11-23  7:20   ` Darren Hart
2013-11-23  0:56 ` [PATCH 5/5] sched,futex: Provide delayed wakeup list Davidlohr Bueso
2013-11-23 11:48   ` Peter Zijlstra
2013-11-23 12:01     ` Peter Zijlstra
2013-11-24  5:25       ` Davidlohr Bueso
2013-11-23  5:55 ` [PATCH 0/5] futex: Wakeup optimizations Darren Hart
2013-11-23  6:35   ` Mike Galbraith
2013-11-23  6:38   ` Davidlohr Bueso
2014-01-12 23:31 [PATCH v6 " Davidlohr Bueso
2014-01-12 23:31 ` [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup Davidlohr Bueso

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