linux-kernel - [PATCH v3 09/10] KVM: arm64: vgic: Ensure the irq refcount is nonzero when taking a ref

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Message-ID: <20240216184153.2714504-10-oliver.upton@linux.dev>
Date: Fri, 16 Feb 2024 18:41:52 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: kvmarm@...ts.linux.dev
Cc: kvm@...r.kernel.org,
	Marc Zyngier <maz@...nel.org>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	linux-kernel@...r.kernel.org,
	Oliver Upton <oliver.upton@...ux.dev>
Subject: [PATCH v3 09/10] KVM: arm64: vgic: Ensure the irq refcount is nonzero when taking a ref

It will soon be possible for get() and put() calls to happen in
parallel, which means in most cases we must ensure the refcount is
nonzero when taking a new reference. Switch to using
vgic_try_get_irq_kref() where necessary, and document the few conditions
where an IRQ's refcount is guaranteed to be nonzero.

Signed-off-by: Oliver Upton <oliver.upton@...ux.dev>
---
 arch/arm64/kvm/vgic/vgic-its.c | 18 ++++++++----------
 arch/arm64/kvm/vgic/vgic.c     |  3 ++-
 2 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 27a7451ad8b9..c3d1bca0b458 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -74,18 +74,11 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 	 * check that we don't add a second list entry with the same LPI.
 	 */
 	oldirq = xa_load(&dist->lpi_xa, intid);
-	if (oldirq) {
+	if (vgic_try_get_irq_kref(oldirq)) {
 		/* Someone was faster with adding this LPI, lets use that. */
 		kfree(irq);
 		irq = oldirq;
 
-		/*
-		 * This increases the refcount, the caller is expected to
-		 * call vgic_put_irq() on the returned pointer once it's
-		 * finished with the IRQ.
-		 */
-		vgic_get_irq_kref(irq);
-
 		goto out_unlock;
 	}
 
@@ -609,8 +602,8 @@ static struct vgic_irq *vgic_its_check_cache(struct kvm *kvm, phys_addr_t db,
 	raw_spin_lock_irqsave(&dist->lpi_list_lock, flags);
 
 	irq = __vgic_its_check_cache(dist, db, devid, eventid);
-	if (irq)
-		vgic_get_irq_kref(irq);
+	if (!vgic_try_get_irq_kref(irq))
+		irq = NULL;
 
 	raw_spin_unlock_irqrestore(&dist->lpi_list_lock, flags);
 
@@ -656,6 +649,11 @@ static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
 	if (cte->irq)
 		__vgic_put_lpi_locked(kvm, cte->irq);
 
+	/*
+	 * The irq refcount is guaranteed to be nonzero while holding the
+	 * its_lock, as the ITE (and the reference it holds) cannot be freed.
+	 */
+	lockdep_assert_held(&its->its_lock);
 	vgic_get_irq_kref(irq);
 
 	cte->db		= db;
diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index 128ae53a0a55..2a288d6c0be7 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -394,7 +394,8 @@ bool vgic_queue_irq_unlock(struct kvm *kvm, struct vgic_irq *irq,
 
 	/*
 	 * Grab a reference to the irq to reflect the fact that it is
-	 * now in the ap_list.
+	 * now in the ap_list. This is safe as the caller must already hold a
+	 * reference on the irq.
 	 */
 	vgic_get_irq_kref(irq);
 	list_add_tail(&irq->ap_list, &vcpu->arch.vgic_cpu.ap_list_head);
-- 
2.44.0.rc0.258.g7320e95886-goog