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

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

Jeremy Fitzhardinge wrote:

> On 09/09/09 09:30, Yang, Xiaowei wrote:
> > 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.
> OK.  And the CPU only guarantees that, without explicit barriers,
> write-read ordering is only maintained between accesses to the same
> memory location, not separate locations?
Exactly! For more details please refer to Chapter 8.2 of Intel SDM 3A.

Thanks,
Xiaowei

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