[PATCH v4 08/10] KVM: arm64: vgic: Rely on RCU protection in vgic_get_lpi()

[Oliver Upton <oliver.upton@linux.dev>]

Stop acquiring the lpi_list_lock in favor of RCU for protecting
the read-side critical section in vgic_get_lpi(). In order for this to
be safe, we also need to be careful not to take a reference on an irq
with a refcount of 0, as it is about to be freed.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/kvm/vgic/vgic.c |  9 ++++-----
 arch/arm64/kvm/vgic/vgic.h | 14 +++++++++++---
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index 3fedc58e663a..76abf3d946fe 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -63,15 +63,14 @@ static struct vgic_irq *vgic_get_lpi(struct kvm *kvm, u32 intid)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
 	struct vgic_irq *irq = NULL;
-	unsigned long flags;
 
-	raw_spin_lock_irqsave(&dist->lpi_list_lock, flags);
+	rcu_read_lock();
 
 	irq = xa_load(&dist->lpi_xa, intid);
-	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);
+	rcu_read_unlock();
 
 	return irq;
 }
diff --git a/arch/arm64/kvm/vgic/vgic.h b/arch/arm64/kvm/vgic/vgic.h
index 8d134569d0a1..f874b9932c5a 100644
--- a/arch/arm64/kvm/vgic/vgic.h
+++ b/arch/arm64/kvm/vgic/vgic.h
@@ -220,12 +220,20 @@ void vgic_v2_vmcr_sync(struct kvm_vcpu *vcpu);
 void vgic_v2_save_state(struct kvm_vcpu *vcpu);
 void vgic_v2_restore_state(struct kvm_vcpu *vcpu);
 
-static inline void vgic_get_irq_kref(struct vgic_irq *irq)
+static inline bool vgic_try_get_irq_kref(struct vgic_irq *irq)
 {
+	if (!irq)
+		return false;
+
 	if (irq->intid < VGIC_MIN_LPI)
-		return;
+		return true;
 
-	kref_get(&irq->refcount);
+	return kref_get_unless_zero(&irq->refcount);
+}
+
+static inline void vgic_get_irq_kref(struct vgic_irq *irq)
+{
+	WARN_ON_ONCE(!vgic_try_get_irq_kref(irq));
 }
 
 void vgic_v3_fold_lr_state(struct kvm_vcpu *vcpu);
-- 
2.44.0.rc0.258.g7320e95886-goog