Re: [PATCH] KVM: Drop kvm_vcpu.ready to squash race where "ready" can get stuck "true"

[Paolo Bonzini <pbonzini@redhat.com>]

On 4/9/26 23:33, Sean Christopherson wrote:
> +static inline bool kvm_vcpu_is_runnable_and_scheduled_out(struct kvm_vcpu *vcpu)
> +{
> +	return READ_ONCE(vcpu->preempted) ||
> +	       (READ_ONCE(vcpu->scheduled_out) &&
> +		READ_ONCE(vcpu->wants_to_run) &&

wants_to_run doesn't seem important here, because blocking will never be 
set outside KVM_RUN (unlike scheduled_out which can be set within any 
vcpu_load/vcpu_put pair, if you're unlucky enough).

> +		READ_ONCE(vcpu->stat.generic.blocking) &&
> +		!kvm_vcpu_is_blocking(vcpu));

If you get here you have done the finish_rcuwait() in kvm_vcpu_block(), 
meaning that you've been already scheduled in, haven't you?  So, you 
would need something like this:

static inline bool kvm_vcpu_is_runnable_and_scheduled_out(struct 
kvm_vcpu *vcpu)
{
	if (READ_ONCE(vcpu->preempted))
		return true;

	if (!READ_ONCE(vcpu->scheduled_out))
		return false;
	if (!READ_ONCE(vcpu->stat.generic.blocking))
		return false;
	return rcuwait_was_woken(kvm_arch_vcpu_get_wait(vcpu));
}

// in rcuwait.h
static inline bool rcuwait_was_woken(struct rcuwait *w)
{
	guard(rcu)();
	struct task_struct *t = rcu_access_pointer(w->task);
	return t && !task_is_runnable(t);
}

Paolo

> +}
> +
>   #ifdef __KVM_HAVE_ARCH_INTC_INITIALIZED
>   /*
>    * returns true if the virtual interrupt controller is initialized and
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 9faf70ccae7a..9f71e32daac5 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -455,7 +455,6 @@ static void kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
>   	kvm_vcpu_set_in_spin_loop(vcpu, false);
>   	kvm_vcpu_set_dy_eligible(vcpu, false);
>   	vcpu->preempted = false;
> -	vcpu->ready = false;
>   	preempt_notifier_init(&vcpu->preempt_notifier, &kvm_preempt_ops);
>   	vcpu->last_used_slot = NULL;
>   
> @@ -3803,7 +3802,6 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_vcpu_halt);
>   bool kvm_vcpu_wake_up(struct kvm_vcpu *vcpu)
>   {
>   	if (__kvm_vcpu_wake_up(vcpu)) {
> -		WRITE_ONCE(vcpu->ready, true);
>   		++vcpu->stat.generic.halt_wakeup;
>   		return true;
>   	}
> @@ -4008,7 +4006,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>   			continue;
>   
>   		vcpu = xa_load(&kvm->vcpu_array, idx);
> -		if (!READ_ONCE(vcpu->ready))
> +		if (!kvm_vcpu_is_runnable_and_scheduled_out(vcpu))
>   			continue;
>   		if (kvm_vcpu_is_blocking(vcpu) && !vcpu_dy_runnable(vcpu))
>   			continue;
> @@ -6393,7 +6391,6 @@ static void kvm_sched_in(struct preempt_notifier *pn, int cpu)
>   	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
>   
>   	WRITE_ONCE(vcpu->preempted, false);
> -	WRITE_ONCE(vcpu->ready, false);
>   
>   	__this_cpu_write(kvm_running_vcpu, vcpu);
>   	kvm_arch_vcpu_load(vcpu, cpu);
> @@ -6408,10 +6405,9 @@ static void kvm_sched_out(struct preempt_notifier *pn,
>   
>   	WRITE_ONCE(vcpu->scheduled_out, true);
>   
> -	if (task_is_runnable(current) && vcpu->wants_to_run) {
> +	if (task_is_runnable(current) && vcpu->wants_to_run)
>   		WRITE_ONCE(vcpu->preempted, true);
> -		WRITE_ONCE(vcpu->ready, true);
> -	}
> +
>   	kvm_arch_vcpu_put(vcpu);
>   	__this_cpu_write(kvm_running_vcpu, NULL);
>   }
> 
> base-commit: b89df297a47e641581ee67793592e5c6ae0428f4