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

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Date:   Thu, 30 Mar 2023 17:56:14 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
CC:     Sean Christopherson <seanjc@...gle.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] KVM: VMX: fix lockdep warning on posted intr wakeup

On Wed, Mar 29, 2023 at 01:51:23PM +0200, Paolo Bonzini wrote:
> On 3/29/23 03:53, Yan Zhao wrote:
> > Yes, there's no actual deadlock currently.
> > 
> > But without fixing this issue, debug_locks will be set to false along
> > with below messages printed. Then lockdep will be turned off and any
> > other lock detections like lockdep_assert_held()... will not print
> > warning even when it's obviously violated.
> 
> Can you use lockdep subclasses, giving 0 to the sched_in path and 1 to the
> sched_out path?

Yes, thanks for the suggestion!
This can avoid this warning of "possible circular locking dependency".

I tried it like this:
- in sched_out path:
  raw_spin_lock_nested(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu), 1);

- in irq and sched_in paths:
  raw_spin_lock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu));

But I have a concern:
If sched_in path removes vcpu A from wakeup list of its previous pcpu A,
and at the mean time, sched_out path adds vcpu B to the wakeup list of
pcpu A, the sched_in and sched_out paths should race for the same
subclass of lock.
But if sched_in path only holds subclass 0, and sched_out path holds
subclass 1, then lockdep would not warn of "possible circular locking
dependency" if someone made a change as below in sched_in path.

if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR) {
            raw_spin_lock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu));
            list_del(&vmx->pi_wakeup_list);
+            raw_spin_lock(&current->pi_lock);
+            raw_spin_unlock(&current->pi_lock);
            raw_spin_unlock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu));
}

While with v3 of this patch (sched_in path holds both out_lock and in_lock),
lockdep is still able to warn about this issue.

Thanks
Yan