linux-kernel - Re: possible deadlock due to irq_set_thread_affinity() calling into the scheduler (was Re: [PATCH v3 38/62] KVM: SVM: Take and hold ir_list

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Message-ID: <CABgObfbijO2k6fe6z03M3u--4ZKXxwPSf2RrasU1=J5rtj9CUg@mail.gmail.com>
Date: Sat, 24 Jan 2026 08:49:40 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Thomas Gleixner <tglx@...nel.org>
Cc: Ankit Soni <Ankit.Soni@....com>, Sean Christopherson <seanjc@...gle.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Joerg Roedel <joro@...tes.org>, 
	David Woodhouse <dwmw2@...radead.org>, Lu Baolu <baolu.lu@...ux.intel.com>, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Sairaj Kodilkar <sarunkod@....com>, Vasant Hegde <vasant.hegde@....com>, 
	Maxim Levitsky <mlevitsk@...hat.com>, Joao Martins <joao.m.martins@...cle.com>, 
	Francesco Lavra <francescolavra.fl@...il.com>, David Matlack <dmatlack@...gle.com>, 
	Naveen Rao <Naveen.Rao@....com>, Crystal Wood <crwood@...hat.com>
Subject: Re: possible deadlock due to irq_set_thread_affinity() calling into
 the scheduler (was Re: [PATCH v3 38/62] KVM: SVM: Take and hold ir_list_lock
 across IRTE updates in IOMMU)

On Thu, Jan 22, 2026 at 7:47 PM Thomas Gleixner <tglx@...nel.org> wrote:
>
> On Wed, Jan 21 2026 at 19:13, Paolo Bonzini wrote:
> > On 1/8/26 22:53, Thomas Gleixner wrote:
> >> Are you still claiming that this is a kernel/irq bug?
> >
> > Not really, I did say I'd like to treat it as a kernel/irq bug...
> > but certainly didn't have hopes high enough to "claim" that.
> > I do think that it's ugly to have locks that are internal,
> > non-leaf and held around callbacks; but people smarter than
> > me have thought about it and you can't call it a bug anyway.
>
> Deep core code has a tendency to be ugly. But if it makes your life
> easier, then these wakeups can be delayed via an irq_work to be outside
> of the lock. That needs some life-time issues to be addressed, but
> should be doable.

Thanks for the suggestion---hopefully it's not needed at all and we
can delay taking the lock in KVM.

Paolo