[PATCH v7 5/6] KVM: x86: Refactor tsc synchronization code

[Paolo Bonzini]

On 02/09/21 21:21, Sean Christopherson wrote:
> 
>> +	if (!matched) {
>> +		...
>> +		spin_lock(&kvm->arch.pvclock_gtod_sync_lock);
>> +		kvm->arch.nr_vcpus_matched_tsc = 0;
>> +	} else if (!already_matched) {
>> +		spin_lock(&kvm->arch.pvclock_gtod_sync_lock);
>> +		kvm->arch.nr_vcpus_matched_tsc++;
>> +	}
>> +
>> +	kvm_track_tsc_matching(vcpu);
>> +	spin_unlock(&kvm->arch.pvclock_gtod_sync_lock);
>
> This unlock is imbalanced if matched and already_matched are both true.  It's not
> immediately obvious that that_can't_  happen, and if it truly can't happen then
> conditionally locking is pointless (because it's not actually conditional).

This is IMO another reason to unify tsc_write_lock and 
pvclock_gtod_sync_lock.  The chances of contention are pretty slim.  As 
soon as I sort out the next -rc3 pull request I'll send out my version 
of Oliver's patches.

Thanks,

Paolo