linux-kernel - Re: [PATCH 2/2] KVM: VMX: Use separate subclasses for PI wakeup lock to squash false positive

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Message-ID: <Z-2Ywg6UK8lLYklA@google.com>
Date: Wed, 2 Apr 2025 13:06:26 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: VMX: Use separate subclasses for PI wakeup lock
 to squash false positive

On Wed, Apr 02, 2025, Yan Zhao wrote:
> On Tue, Apr 01, 2025 at 08:47:27AM -0700, Sean Christopherson wrote:
> > I.e. lockdep sees AB+BC ordering for schedule out, and CA ordering for
> > wakeup, and complains about the A=>C versus C=>A inversion.  In practice,
> > deadlock can't occur between schedule out and the wakeup handler as they
> > are mutually exclusive.  The entirely of the schedule out code that runs
> > with the problematic scheduler locks held, does so with IRQs disabled,
> > i.e. can't run concurrently with the wakeup handler.
> > 
> > Use a subclass instead disabling lockdep entirely, and tell lockdep that
> Paolo initially recommended utilizing the subclass.
> Do you think it's good to add his suggested-by tag?

Sure.

> BTW: is it necessary to state the subclass assignment explicitly in the
> patch msg? e.g.,
> 
> wakeup handler: subclass 0
> sched_out: subclass 1
> sched_in: subclasses 0 and 1

Yeah, explicitly stating the effectively rules would be helpful.  If those are
the only issues, I'll just fixup the changelog when applying.