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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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?

    J

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