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Date: Tue, 26 Nov 2013 14:53:22 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Darren Hart <dvhart@linux.intel.com>
cc: Davidlohr Bueso <davidlohr@hp.com>,
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Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup
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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