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

Avi Kivity
2010-02-18T09:45:34Z

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.

--
error compiling committee.c: too many arguments to function