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: <87sebxtrgp.ffs@tglx>
Date: Thu, 22 Jan 2026 19:47:18 +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 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,

        tglx