Re: [patch] x86: kvm: Convert i8254/i8259 locks to raw_spinlocks

Jan Kiszka
2010-02-18T09:50:19Z

Avi Kivity wrote:
> On 02/18/2010 11:40 AM, Jan Kiszka wrote:
>>> Meanwhile, if anyone has any idea how to kill this lock, I'd love to see it.
>>>
>>>
>> What concurrency does it resolve in the end? On first glance, it only
>> synchronize the fiddling with pre-VCPU request bits, right? What forces
>> us to do this? Wouldn't it suffice to disable preemption (thus
>> migration) and the let concurrent requests race for setting the bits? I
>> mean if some request bit was already set on entry, we don't include the
>> related VCPU in smp_call_function_many anyway.
>>
>
> It's more difficult.
>
> vcpu 0: sets request bit on vcpu 2
> vcpu 1: test_and_set request bit on vcpu 2, returns already set
> vcpu 1: returns
> vcpu 0: sends IPI
> vcpu 0: returns
>
> so vcpu 1 returns before the IPI was performed. If the request was a
> tlb flush, for example, vcpu 1 may free a page that is still in vcpu 2's
> tlb.

So the requests bits we are interested in are exclusively set in this
function under requests_lock?

Jan

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