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

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Message-ID: <55BA2E68.9030207@redhat.com>
Date:	Thu, 30 Jul 2015 16:02:16 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.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



On 30/07/2015 15:57, 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).
>>>>> +	 */
> 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.

What you are suggesting would go in kvm_vm_ioctl_create_vcpu, e.g.
rewriting:

	/* Pairs with smp_rmb() in kvm_get_vcpu.  */
	smp_wmb();

like this:

	/*
	 * Pairs with smp_rmb() in kvm_get_vcpu.  Write kvm->vcpus
	 * before kvm->online_vcpu's incremented value.
	 */
	smp_wmb();

Instead, kvm_get_cpu has:

	/*
	 * 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();

which already describes which loads are serialized here.

Is this correct?

Paolo
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