[PATCH] drm/mst: use kref_get_unless_zero for looking up mst branch device

[Dave Airlie]

From: Dave Airlie <airlied at redhat.com>

The backtrace below shows that while we tear down a branch device,
when we remove the connector, we can attempt to disable MST on the
port we are tearing down, since the port is going away, there it
has a refcount of 0 already, so the validated ref lookup is trying
to kref_get again and failing.

Use kref_get_unless_zero and just don't return anything if we are
looking up the object we are currently also cleaning.

 [<ffffffffa00cc262>] drm_dp_mst_get_validated_mstb_ref_locked+0x92/0xa0 [drm_kms_helper]
 [<ffffffffa00cc211>] drm_dp_mst_get_validated_mstb_ref_locked+0x41/0xa0 [drm_kms_helper]
 [<ffffffffa00cc2aa>] drm_dp_get_validated_mstb_ref+0x3a/0x60 [drm_kms_helper]
 [<ffffffffa00cc2fb>] drm_dp_payload_send_msg.isra.14+0x2b/0x100 [drm_kms_helper]
 [<ffffffffa00cc547>] drm_dp_update_payload_part1+0x177/0x360 [drm_kms_helper]
 [<ffffffffa015c52e>] intel_mst_disable_dp+0x3e/0x80 [i915]
 [<ffffffffa013d60b>] haswell_crtc_disable+0x1cb/0x340 [i915]
 [<ffffffffa0136739>] intel_crtc_control+0x49/0x100 [i915]
 [<ffffffffa0136857>] intel_crtc_update_dpms+0x67/0x80 [i915]
 [<ffffffffa013fa59>] intel_connector_dpms+0x59/0x70 [i915]
 [<ffffffffa015c752>] intel_dp_destroy_mst_connector+0x32/0xc0 [i915]
 [<ffffffffa00cb44b>] drm_dp_destroy_port+0x6b/0xa0 [drm_kms_helper]
 [<ffffffffa00cb588>] drm_dp_destroy_mst_branch_device+0x108/0x130 [drm_kms_helper]
 [<ffffffffa00cb3cd>] drm_dp_port_teardown_pdt+0x3d/0x50 [drm_kms_helper]
 [<ffffffffa00cdb79>] drm_dp_mst_handle_up_req+0x499/0x540 [drm_kms_helper]
 [<ffffffff810d9ead>] ? trace_hardirqs_on_caller+0x15d/0x200
 [<ffffffffa00cdc73>] drm_dp_mst_hpd_irq+0x53/0xa00 [drm_kms_helper]
 [<ffffffffa00c7dfb>] ? drm_dp_dpcd_read+0x1b/0x20 [drm_kms_helper]
 [<ffffffffa0153ed8>] ? intel_dp_dpcd_read_wake+0x38/0x70 [i915]
 [<ffffffffa015a225>] intel_dp_check_mst_status+0xb5/0x250 [i915]
 [<ffffffffa015ac71>] intel_dp_hpd_pulse+0x181/0x210 [i915]
 [<ffffffffa01104f6>] i915_digport_work_func+0x96/0x120 [i915]

Signed-off-by: Dave Airlie <airlied at redhat.com>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index ce1113c..f703a5b 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -882,8 +882,10 @@ static struct drm_dp_mst_branch *drm_dp_mst_get_validated_mstb_ref_locked(struct
 	struct drm_dp_mst_port *port;
 	struct drm_dp_mst_branch *rmstb;
 	if (to_find == mstb) {
-		kref_get(&mstb->kref);
-		return mstb;
+		if (!kref_get_unless_zero(&mstb->kref))
+			return NULL;
+		else
+			return mstb;
 	}
 	list_for_each_entry(port, &mstb->ports, next) {
 		if (port->mstb) {
-- 
2.1.0