linux-kernel - Re: [PATCH] KVM: x86: fix deadlock for KVM_XEN_EVTCHN

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Message-ID: <747f1eac-c620-d7cb-86a1-a2c575cd2ef9@rbox.co>
Date:   Fri, 24 Feb 2023 19:18:02 +0100
From:   Michal Luczaj <mhal@...x.co>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH] KVM: x86: fix deadlock for KVM_XEN_EVTCHN_RESET

On 28/12/2022 12:04, Paolo Bonzini wrote:
> While KVM_XEN_EVTCHN_RESET is usually called with no vCPUs running,
> if that happened it could cause a deadlock.  This is due to
> kvm_xen_eventfd_reset() doing a synchronize_srcu() inside
> a kvm->lock critical section.
>
> [...]
>
> +	/*
> +	 * Because synchronize_srcu() cannot be called inside the
> +	 * critical section, first collect all the evtchnfd objects
> +	 * in an array as they are removed from evtchn_ports.
> +	 */

With the recent changes regarding the locking order (locking.rst:
"synchronize_srcu(&kvm->srcu) is called inside critical sections for kvm->lock,
vcpu->mutex and kvm->slots_lock"), is this comment still valid?

Or is there a rule that forbids synchronize_srcu() under the newly introduced
kvm->arch.xen.xen_lock?

thanks,
Michal