From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: [PATCH] Fix memory order issue inside pv spinlock
Date: Wed, 09 Sep 2009 12:02:13 -0700
Message-ID: <4AA7FBB5.3000204@goop.org>
References: <4AA4B8FF.9070905@intel.com> <4AA6D3CF.70905@goop.org>
	<4AA7D808.6010108@intel.com>
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To: "Yang, Xiaowei" <xiaowei.yang@intel.com>
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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