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Mon, 22 Dec 2025 06:09:21 -0800 (PST) Received: from [192.168.10.48] ([151.95.145.106]) by smtp.googlemail.com with ESMTPSA id ffacd0b85a97d-4324eaa2beasm21799471f8f.33.2025.12.22.06.09.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 22 Dec 2025 06:09:20 -0800 (PST) Message-ID: <42513cb3-3c2e-4aa8-b748-23b6656a5096@redhat.com> Date: Mon, 22 Dec 2025 15:09:13 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: 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) To: Ankit Soni , Sean Christopherson , Marc Zyngier Cc: Oliver Upton , Joerg Roedel , David Woodhouse , Lu Baolu , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, iommu@lists.linux.dev, linux-kernel@vger.kernel.org, Sairaj Kodilkar , Vasant Hegde , Maxim Levitsky , Joao Martins , Francesco Lavra , David Matlack , Naveen Rao References: <20250611224604.313496-2-seanjc@google.com> <20250611224604.313496-40-seanjc@google.com> From: Paolo Bonzini Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251222_060928_361413_F727DEF0 X-CRM114-Status: GOOD ( 16.09 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 12/22/25 10:16, Ankit Soni wrote: > ====================================================== > WARNING: possible circular locking dependency detected > 6.19.0-rc2 #20 Tainted: G E > ------------------------------------------------------ > CPU 58/KVM/28597 is trying to acquire lock: > ff12c47d4b1f34c0 (&irq_desc_lock_class){-.-.}-{2:2}, at: __irq_get_desc_lock+0x58/0xa0 > > but task is already holding lock: > ff12c49b28552110 (&svm->ir_list_lock){....}-{2:2}, at: avic_pi_update_irte+0x147/0x270 [kvm_amd] > > which lock already depends on the new lock. > > Chain exists of: > &irq_desc_lock_class --> &rq->__lock --> &svm->ir_list_lock > > Possible unsafe locking scenario: > > CPU0 CPU1 > ---- ---- > lock(&svm->ir_list_lock); > lock(&rq->__lock); > lock(&svm->ir_list_lock); > lock(&irq_desc_lock_class); > > *** DEADLOCK *** > > So lockdep sees: > > &irq_desc_lock_class -> &rq->__lock -> &svm->ir_list_lock > > while avic_pi_update_irte() currently holds svm->ir_list_lock and then > takes irq_desc_lock via irq_set_vcpu_affinity(), which creates the > potential inversion. > > - Is this lockdep warning expected/benign in this code path, or does it > indicate a real potential deadlock between svm->ir_list_lock and > irq_desc_lock with AVIC + irq_bypass + VFIO? I'd treat it as a potential (if unlikely) deadlock: (a) irq_set_thread_affinity triggers the scheduler via wake_up_process, while irq_desc->lock is taken (b) the scheduler calls into KVM with rq_lock taken, and KVM uses ir_list_lock within __avic_vcpu_load/__avic_vcpu_put (c) KVM wants to block scheduling for a while and uses ir_list_lock for that purpose, but then takes irq_set_vcpu_affinity takes irq_desc->lock. I don't think there's an alternative choice of lock for (c); and there's no easy way to pull the irq_desc->lock out of the IRQ subsystem--in fact the stickiness of the situation comes from rq->rq_lock and irq_desc->lock being both internal and not leaf. 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 to drop the wake_up_process(). The only cost is extra latency on the next interrupt after an affinity change. diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c index 8b1b4c8a4f54..fc135bd079a4 100644 --- a/kernel/irq/manage.c +++ b/kernel/irq/manage.c @@ -189,14 +189,10 @@ static void irq_set_thread_affinity(struct irq_desc *desc) struct irqaction *action; for_each_action_of_desc(desc, action) { - if (action->thread) { + if (action->thread) set_bit(IRQTF_AFFINITY, &action->thread_flags); - wake_up_process(action->thread); - } - if (action->secondary && action->secondary->thread) { + if (action->secondary && action->secondary->thread) set_bit(IRQTF_AFFINITY, &action->secondary->thread_flags); - wake_up_process(action->secondary->thread); - } } } Marc, what do you think? Paolo