linux-kernel - Re: [PATCH 2/2] KVM: Protect vCPU's "last run PID" with rwlock, not RCU

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Message-ID: <ZrZafa66bRxoVc0Z@linux.dev>
Date: Fri, 9 Aug 2024 11:05:49 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Marc Zyngier <maz@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Steve Rutherford <srutherford@...gle.com>
Subject: Re: [PATCH 2/2] KVM: Protect vCPU's "last run PID" with rwlock, not
 RCU

On Tue, Aug 06, 2024 at 04:59:03PM -0700, Sean Christopherson wrote:
> > Can you nest this lock inside of the vcpu->mutex acquisition in
> > kvm_vm_ioctl_create_vcpu() so lockdep gets the picture?
> 
> I don't think that's necessary.  Commit 42a90008f890 ("KVM: Ensure lockdep knows
> about kvm->lock vs. vcpu->mutex ordering rule") added the lock+unlock in
> kvm_vm_ioctl_create_vcpu() purely because actually taking vcpu->mutex inside
> kvm->lock is rare, i.e. lockdep would be unable to detect issues except for very
> specific VM types hitting very specific flows.

I don't think the perceived rarity matters at all w/ this. Beyond the
lockdep benefits, it is a self-documenting way to describe lock ordering.
Dunno about you, but I haven't kept up with locking.rst at all :)

Having said that, an inversion would still be *very* obvious, as it
would be trying to grab a mutex while holding a spinlock...

-- 
Thanks,
Oliver