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Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq

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2016-02-18 19:04+0100, Paolo Bonzini:
> On 17/02/2016 20:14, Radim Krčmář wrote:
>> -	value = atomic_dec_return(&ps->pending);
>> -	if (value < 0)
>> -		/* spurious acks can be generated if, for example, the
>> -		 * PIC is being reset.  Handle it gracefully here
>> -		 */
>> -		atomic_inc(&ps->pending);
>> -	else if (value > 0 && ps->reinject)
>> -		/* in this case, we had multiple outstanding pit interrupts
>> -		 * that we needed to inject.  Reinject
>> -		 */
>> +	if (atomic_dec_if_positive(&ps->pending) > 0 && ps->reinject)
>>  		queue_kthread_work(&ps->pit->worker, &ps->pit->expired);
> 
> Here it would have made sense to do already
> 
> 	if (!ps->reinject) {
> 		WARN_ON_ONCE(ps->pending || !ps->irq_ack);
> 		return;
> 	}

I will add the WARN_ON when removing discard notifiers.

> 	spin_lock(...)
> 	if (atomic_dec_if_positive(&ps->pending) > 0)
> 		queue_kthread_work(...);
> 	ps->irq_ack = 1;
> 	spin_unlock(...)
> 	
> because ps->pending is only ever nonzero, and irq_ack is only ever zero,
> if ps->reinject.

(Well, userspace can switch between policies at runtime.)

>                   Not a big deal since the ack notifier is going to
> disappear altogether for the discard policy, but the nice thing is that
> it lets you remove the ack notifier earlier and disentangle a bit more
> discard mode.
> 
> So if you want for v3 you can reorder the patches like this:

The end result is going to be identical.  I had a version that did
something similar and it was pretty tangled as well -- I wanted to
remove useless locks before re-using one for the ioctls.
(We need the protection earlier, because userspace can control notifiers
 while PIT is still being initialized.  And removing the lock had
 dependencies.)

> 
> - patch 1, same
> 
> - patch 2, what is outlined above
> 
> - patch 3, remove ack notifier for discard

I agree that current ordering looks weird.  The dependency tree looked
like this in my mind:

-[1/14]
 ,-[2/14]
-[4/14]
 ,-[3/14]
 |   ,-[5/14]
 | ,-[6/14]
 +-[7/14]
-[8/14]
-[9-14/14]

I added [2-4/14] early (and a bit out of order), because it made diffs
shorter.  Dependency on [7/14] can dropped with correct mutexing inside
initialization, so the v3 order would be:

-[1/14]
 ,-[3/14]
-[8/14]
 ,-[2/14]
-[4/14]
   ,-[5/14]
 ,-[6/14]
-[7/14]
-[9-14/14]

With [8/14] (remove ack notifier for discard) as third.

Would that be ok?

> - patch 4..14 the rest
> 
> Paolo

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[PATCH v2 00/14] KVM: x86: change PIT discard policy and untangle related code Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
    Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:10 +0100
      Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq Paolo Bonzini <pbonzini@redhat.com> - 2016-02-19 17:00 +0100
      Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 17:00 +0100
  [PATCH v2 14/14] KVM: x86: move PIT timer function initialization Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 05/14] KVM: x86: tone down WARN_ON pit.state_lock Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 06/14] KVM: x86: pass struct kvm_pit instead of kvm in PIT Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 09/14] KVM: x86: refactor kvm_create_pit Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 04/14] KVM: x86: use atomic_t instead of pit.inject_lock Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 13/14] KVM: x86: don't assume layout of kvm_kpit_state Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
    Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 17:20 +0100
      Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-18 18:00 +0100
        Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 18:40 +0100
          Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:00 +0100
            Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 15:50 +0100
    Re: [PATCH v2 01/14] KVM: x86: change PIT discard tick policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:50 +0100
  [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
    Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:10 +0100
    Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:10 +0100
      Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 16:10 +0100
        Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Paolo Bonzini <pbonzini@redhat.com> - 2016-02-19 16:10 +0100
          Re: [PATCH v2 08/14] KVM: x86: remove notifiers from PIT discard  policy Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 16:10 +0100
  [PATCH v2 10/14] KVM: x86: refactor kvm_free_pit Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 11/14] KVM: x86: remove pit and kvm from kvm_kpit_state Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
  [PATCH v2 07/14] KVM: x86: remove unnecessary uses of PIT state lock Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:20 +0100
    Re: [PATCH v2 07/14] KVM: x86: remove unnecessary uses of PIT state  lock Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:10 +0100
      Re: [PATCH v2 07/14] KVM: x86: remove unnecessary uses of PIT state  lock Radim Krčmář <rkrcmar@redhat.com> - 2016-02-19 15:50 +0100
  [PATCH v2 03/14] KVM: x86: add kvm_pit_reset_reinject Radim Krčmář <rkrcmar@redhat.com> - 2016-02-17 20:30 +0100
  Re: [PATCH v2 00/14] KVM: x86: change PIT discard policy and untangle  related code Paolo Bonzini <pbonzini@redhat.com> - 2016-02-18 19:20 +0100

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