linux-kernel - Re: [PATCH v3] KVM: LAPIC: Fix reentrancy issues with preempt notifiers

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Message-ID: <18bcec8d-07ab-d523-5569-46319a04a1dd@redhat.com>
Date:   Tue, 25 Jul 2017 09:25:12 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v3] KVM: LAPIC: Fix reentrancy issues with preempt
 notifiers

On 25/07/2017 00:43, Wanpeng Li wrote:
> -	WARN_ON(!apic->lapic_timer.hv_timer_in_use);
> +	preempt_disable();
> +	/* The preempt notifier has called apic_timer_expired already */
> +	if (!apic->lapic_timer.hv_timer_in_use)
> +		goto out;
>  	WARN_ON(swait_active(&vcpu->wq));
>  	cancel_hv_timer(apic);
>  	apic_timer_expired(apic);
> +out:
> +	preempt_enable();

If apic_timer_expired was called, and the timer is in periodic mode, it
has already set the hv timer.  Should the out label be really at the end
of the function, after the timer is restarted?  Otherwise you can call
advance_periodic_timer_expiration twice.

Thanks,

Paolo

>  
>  	if (apic_lvtt_period(apic) && apic->lapic_timer.period) {
>  		advance_periodic_target_expiration(apic);
> @@ -1582,9 +1588,11 @@ void kvm_lapic_switch_to_sw_timer(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_lapic *apic = vcpu->arch.apic;
>  
> +	preempt_disable();
>  	/* Possibly the TSC deadline timer is not enabled yet */
>  	if (apic->lapic_timer.hv_timer_in_use)
>  		start_sw_timer(apic);
> +	preempt_enable();
>  }
>  EXPORT_SYMBOL_GPL(kvm_lapic_switch_to_sw_timer);