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: <87pl7jsrdg.ffs@tglx>
Date: Thu, 08 Jan 2026 22:53:47 +0100
From: Thomas Gleixner <tglx@...nel.org>
To: Paolo Bonzini <pbonzini@...hat.com>, Ankit Soni <Ankit.Soni@....com>,
 Sean Christopherson <seanjc@...gle.com>, Marc Zyngier <maz@...nel.org>
Cc: 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 08 2026 at 22:28, Thomas Gleixner wrote:
> On Mon, Dec 22 2025 at 15:09, Paolo Bonzini wrote:
>> Of the three, the most sketchy is (a); notably, __setup_irq() calls
>> wake_up_process outside desc->lock.  Therefore I'd like so much to treat
>> it as a kernel/irq/ bug; and the simplest (perhaps too simple...) fix is
>
> It's not more sketchy than VIRT assuming that it can do what it wants
> under rq->lock. :)

And just for the record, that's not the only place in the irq core which
has that lock chain.

irq_set_affinity_locked()       // invoked with desc::lock held
   if (desc->affinity_notify)
      schedule_work()           // Ends up taking rq::lock

and that's the case since cd7eab44e994 ("genirq: Add IRQ affinity
notifiers"), which was added 15 years ago.

Are you still claiming that this is a kernel/irq bug?

Thanks,

        tglx