Re: [PATCH RFC] KVM: Fix race in apic->pending_events processing

[Paolo Bonzini <pbonzini@redhat.com>]

Il 28/05/2013 17:00, Gleb Natapov ha scritto:
> On Tue, May 28, 2013 at 03:48:58PM +0200, Paolo Bonzini wrote:
>> Il 28/05/2013 14:56, Gleb Natapov ha scritto:
>>>>>  		else
>>>>>  			vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
>>>>>  	}
>>>>> -	if (test_and_clear_bit(KVM_APIC_SIPI, &apic->pending_events) &&
>>>>> +	/*
>>>>> +	 * Note that we may get another INIT+SIPI sequence right here; process
>>>>> +	 * the INIT first.  Assumes that there are only KVM_APIC_INIT/SIPI.
>>>>> +	 */
>>>>> +	if (cmpxchg(&apic->pending_events, KVM_APIC_SIPI, 0) == KVM_APIC_SIPI &&
>>>>>  	    vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
>>> Because pending_events can be INIT/SIPI at this point and it should be
>>> interpreted as: do SIPI and ignore INIT (atomically).
>>
>> My patch does "do another INIT (which will have no effect) and do SIPI 
>> after that INIT", which is different but has almost the same effect.  
>> If pending_events is INIT/SIPI, it ignores the SIPI for now and lets 
>> the next iteration of kvm_apic_accept_events do both.  The difference 
>> would be that in a carefully-timed sequence of interrupts
>>
> You assume that the next processing will actually happen, but this is
> not necessary the case.

Why not?  The INIT and SIPI that have just been sent have kicked the
VCPU again.

>> But then, there's another way to mask INITs in the wait-for-SIPI 
>> state.  Considering that KVM_MP_STATE_INIT_RECEIVED is really a 
>> wait-for-SIPI, you can do:
>>
> Haven't checked it for races (especially races between multiple CPUS
> sending INIT), but looks more complicated to me.

Ok, let's go with yours.

Paolo