linux-kernel - Re: [PATCH 2/4] KVM: document memory barriers for kvm->vcpus/kvm->online

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Date:	Thu, 30 Jul 2015 15:57:02 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
CC:	mtosatti@...hat.com, srutherford@...gle.com, rkrcmar@...hat.com
Subject: Re: [PATCH 2/4] KVM: document memory barriers for kvm->vcpus/kvm->online_vcpus

Am 30.07.2015 um 14:46 schrieb Paolo Bonzini:
> 
> 
> On 30/07/2015 13:40, Christian Borntraeger wrote:
>>>> +	/* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu, in case
>>>> +	 * the caller has read kvm->online_vcpus before (as is the case
>>>> +	 * for kvm_for_each_vcpu, for example).
>>>> +	 */
>>>>  	smp_rmb();
>> Hmmm, wouldnt something like smp_mb__after_atomic
>>
>>>>  	return kvm->vcpus[i];
>>>>  }
>>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>>> index 8dc4828f623f..093b3d10b411 100644
>>>> --- a/virt/kvm/kvm_main.c
>>>> +++ b/virt/kvm/kvm_main.c
>>>> @@ -2206,6 +2206,8 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
>>>>  	}
>>>>
>>>>  	kvm->vcpus[atomic_read(&kvm->online_vcpus)] = vcpu;
>> and  smp_mb__before_atomic
>>
>> be the better function?
>>
>>
>>>> +
>>>> +	/* Pairs with smp_rmb() in kvm_get_vcpu.  */
>>>>  	smp_wmb();
>>>>  	atomic_inc(&kvm->online_vcpus);
> 
> The difference between smp_rmb/smp_wmb on one side, and smp_mb on the
> other side, is that smp_mb protects previous stores from subsequent loads.
> 
> smp_mb__before_atomic would be overkill for atomic_inc, where there's no
> race involving the read of online_vcpus (there's only one writer and
> it's protected by kvm->lock).  We only care about ordering the
> kvm->vcpus store with the kvm->online_vcpus store. 

Ok, makes sense, if we only want to order these two stores.

But then the comment

>>>> +	/* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu, in case
>>>> +	 * the caller has read kvm->online_vcpus before (as is the case
>>>> +	 * for kvm_for_each_vcpu, for example).
>>>> +	 */

is somewhat distracting because of "read" and "before". So something like

 /* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu, to serialize the setting
of kvm->vcpus and setting kvm->online_vcpus....

might be better.


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