linux-kernel - Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack

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Message-ID: <20160219155150.GC2456@potion.brq.redhat.com>
Date:	Fri, 19 Feb 2016 16:51:51 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Yuki Shibuya <shibuya.yk@...s.nec.co.jp>
Subject: Re: [PATCH v2 02/14] KVM: x86: simplify atomics in kvm_pit_ack_irq

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