linux-kernel - Re: [PATCH v3] KVM: VMX: fix lockdep warning on posted intr wakeup

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Date:   Fri, 24 Mar 2023 16:13:37 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com
Subject: Re: [PATCH v3] KVM: VMX: fix lockdep warning on posted intr wakeup

On Mon, Mar 13, 2023, Yan Zhao wrote:
> The lock ordering after this patch are:
> - &p->pi_lock --> &rq->__lock -->
>   &per_cpu(wakeup_vcpus_on_cpu_lock_out, cpu)
> - &per_cpu(wakeup_vcpus_on_cpu_lock_in, cpu) -->
>   &per_cpu(wakeup_vcpus_on_cpu_lock_out, cpu)
> - &per_cpu(wakeup_vcpus_on_cpu_lock_in, cpu) --> &p->pi_lock
> 
> Currently, &rq->__lock is not held in "path sched_in".
> However, if in future "path sched_in" takes &p->pi_lock or &rq->__lock,
> lockdep is able to detect and warn in that case.
> 
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> [sean: path sched_out and path irq does not race, path sched_in does not
> take &rq->__lock]

But there's no actual deadlock, right?  I have zero interest in fixing a lockdep
false positive by making functional changes to KVM.  I am definitely open to making
changes to somehow let lockdep know what's going on, but complicating KVM's actual
functionality is too much.