linux-kernel - [PATCH v3 02/13] KVM: Acquire SCRU lock outside of irqfds.lock during assignment

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Message-ID: <20250522235223.3178519-3-seanjc@google.com>
Date: Thu, 22 May 2025 16:52:12 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: "K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, 
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, 
	Juergen Gross <jgross@...e.com>, Stefano Stabellini <sstabellini@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Shuah Khan <shuah@...nel.org>, 
	Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org, 
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	kvmarm@...ts.linux.dev, K Prateek Nayak <kprateek.nayak@....com>, 
	David Matlack <dmatlack@...gle.com>
Subject: [PATCH v3 02/13] KVM: Acquire SCRU lock outside of irqfds.lock during assignment

Acquire SRCU outside of irqfds.lock so that the locking is symmetrical,
and add a comment explaining why on earth KVM holds SRCU for so long.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 virt/kvm/eventfd.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 39e42b19d9f7..42c02c35e542 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -401,6 +401,18 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 	 */
 	init_waitqueue_func_entry(&irqfd->wait, irqfd_wakeup);
 
+	/*
+	 * Set the irqfd routing and add it to KVM's list before registering
+	 * the irqfd with the eventfd, so that the routing information is valid
+	 * and stays valid, e.g. if there are GSI routing changes, prior to
+	 * making the irqfd visible, i.e. before it might be signaled.
+	 *
+	 * Note, holding SRCU ensures a stable read of routing information, and
+	 * also prevents irqfd_shutdown() from freeing the irqfd before it's
+	 * fully initialized.
+	 */
+	idx = srcu_read_lock(&kvm->irq_srcu);
+
 	spin_lock_irq(&kvm->irqfds.lock);
 
 	ret = 0;
@@ -409,11 +421,9 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 			continue;
 		/* This fd is used for another irq already. */
 		ret = -EBUSY;
-		spin_unlock_irq(&kvm->irqfds.lock);
-		goto fail;
+		goto fail_duplicate;
 	}
 
-	idx = srcu_read_lock(&kvm->irq_srcu);
 	irqfd_update(kvm, irqfd);
 
 	list_add_tail(&irqfd->list, &kvm->irqfds.items);
@@ -449,6 +459,9 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
 	srcu_read_unlock(&kvm->irq_srcu, idx);
 	return 0;
 
+fail_duplicate:
+	spin_unlock_irq(&kvm->irqfds.lock);
+	srcu_read_unlock(&kvm->irq_srcu, idx);
 fail:
 	if (irqfd->resampler)
 		irqfd_resampler_shutdown(irqfd);
-- 
2.49.0.1151.ga128411c76-goog