From: Christoffer Dall <cdall@xxxxxxxxxx>
Getting a per-CPU variable requires a non-preemptible context and we
were relying on a normal spinlock to disable preemption as well. This
asusmption breaks with PREEMPT_RT and was observed on v4.9 using
PREEMPT_RT.
This change moves the spinlock tighter around the critical section
accessing the IRQ structure protected by the lock and uses a separate
preemption disabled section for determining the requesting VCPU. There
should be no change in functionality of performance degradation on
non-RT.
Fixes: 370a0ec18199 ("KVM: arm/arm64: Let vcpu thread modify its own active state")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Jintack Lim <jintack@xxxxxxxxxxxxxxx>
Reported-by: Hemanth Kumar <hemk976@xxxxxxxxx>
Signed-off-by: Christoffer Dall <cdall@xxxxxxxxxx>
---
virt/kvm/arm/vgic/vgic-mmio.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
index c1e4bdd..7377f97 100644
--- a/virt/kvm/arm/vgic/vgic-mmio.c
+++ b/virt/kvm/arm/vgic/vgic-mmio.c
@@ -181,7 +181,6 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
bool new_active_state)
{
struct kvm_vcpu *requester_vcpu;
- spin_lock(&irq->irq_lock);
/*
* The vcpu parameter here can mean multiple things depending on how
@@ -195,8 +194,19 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
* NULL, which is fine, because we guarantee that no VCPUs are running
* when accessing VGIC state from user space so irq->vcpu->cpu is
* always -1.
+ *
+ * We have to temporarily disable preemption to read the per-CPU
+ * variable. It doesn't matter if we actually get preempted
+ * after enabling preemption because we only need to figure out if
+ * this thread is a running VCPU thread, and in that case for which
+ * VCPU. If we're migrated the preempt notifiers will migrate the
+ * running VCPU pointer with us.
*/
+ preempt_disable();
requester_vcpu = kvm_arm_get_running_vcpu();
+ preempt_enable();
+
+ spin_lock(&irq->irq_lock);
/*
* If this virtual IRQ was written into a list register, we
--
2.7.4