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

Oliver Upton
2024-02-16T18:42:24Z

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@linux.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