[Xen-devel] Re: [PATCH] Fix memory order issue inside pv spinlock

["Yang, Xiaowei" <xiaowei.yang@xxxxxxxxx>]

Jeremy Fitzhardinge wrote:
> On 09/07/09 00:40, Yang, Xiaowei wrote:
> > barrier() can't prevent reads after it not being reordered with older
> > writes to different locates before it. Because of it, I can't bring up
> > > 4 HVM guests on one SMP machine. Use mb() instead.
> Which read is happening too early?  Is it "xl->spinners"?  How does it fail?
Yes. If read of xl->spinners happens earlier than write 0 to xl->lock, 
notifications to wake up other spinners can be omitted incorrectly, resulting in 
others polling indefinitely (because of poll evtchn not pending) with the lock 
is uncontended.
Thanks,
xiaowei

> Thanks,
>     J
>
> > Signed-off-by: Yang Xiaowei <xiaowei.yang@xxxxxxxxx>
> >
> > diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> > index 5601506..9dee5f8 100644
> > --- a/arch/x86/xen/spinlock.c
> > +++ b/arch/x86/xen/spinlock.c
> > @@ -324,7 +325,7 @@ static void xen_spin_unlock(struct raw_spinlock
> > *lock)
> >         xl->lock = 0;           /* release lock */
> >
> >         /* make sure unlock happens before kick */
> > -       barrier();
> > +       mb();
> >
> >         if (unlikely(xl->spinners))
> >                 xen_spin_unlock_slow(xl);
> >
> > Thanks,
> > Xiaowei

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