linux-kernel - [PATCH 2/2] drm/dp_mst: Fix clearing payload state on topology disable

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Date:   Fri, 17 Jan 2020 17:47:49 -0500
From:   Lyude Paul <lyude@...hat.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     Sean Paul <sean@...rly.run>, Wayne Lin <Wayne.Lin@....com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] drm/dp_mst: Fix clearing payload state on topology disable

The issues caused by:

64e62bdf04ab ("drm/dp_mst: Remove VCPI while disabling topology mgr")

Prompted me to take a closer look at how we clear the payload state in
general when disabling the topology, and it turns out there's actually
two subtle issues here.

The first is that we're not grabbing &mgr.payload_lock when clearing the
payloads in drm_dp_mst_topology_mgr_set_mst(). Seeing as the canonical
lock order is &mgr.payload_lock -> &mgr.lock (because we always want
&mgr.lock to be the inner-most lock so topology validation always
works), this makes perfect sense. It also means that -technically- there
could be racing between someone calling
drm_dp_mst_topology_mgr_set_mst() to disable the topology, along with a
modeset occurring that's modifying the payload state at the same time.

The second is the more obvious issue that Wayne Lin discovered, that
we're not clearing proposed_payloads when disabling the topology.

I actually can't see any obvious places where the racing caused by the
first issue would break something, and it could be that some of our
higher-level locks already prevent this by happenstance, but better safe
then sorry. So, let's make it so that drm_dp_mst_topology_mgr_set_mst()
first grabs &mgr.payload_lock followed by &mgr.lock so that we never
race when modifying the payload state. Then, we also clear
proposed_payloads to fix the original issue of enabling a new topology
with a dirty payload state. This doesn't clear any of the drm_dp_vcpi
structures, but those are getting destroyed along with the ports anyway.

Cc: Sean Paul <sean@...rly.run>
Cc: Wayne Lin <Wayne.Lin@....com>
Signed-off-by: Lyude Paul <lyude@...hat.com>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 89c2a7505cbd..58287f4c1baf 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -3483,6 +3483,7 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms
 	int ret = 0;
 	struct drm_dp_mst_branch *mstb = NULL;

+	mutex_lock(&mgr->payload_lock);
 	mutex_lock(&mgr->lock);
 	if (mst_state == mgr->mst_state)
 		goto out_unlock;
@@ -3541,7 +3542,10 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms
 		/* this can fail if the device is gone */
 		drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL, 0);
 		ret = 0;
-		memset(mgr->payloads, 0, mgr->max_payloads * sizeof(struct drm_dp_payload));
+		memset(mgr->payloads, 0,
+		       mgr->max_payloads * sizeof(struct drm_dp_payload));
+		memset(mgr->proposed_vcpis, 0,
+		       mgr->max_payloads * sizeof(void*));
 		mgr->payload_mask = 0;
 		set_bit(0, &mgr->payload_mask);
 		mgr->vcpi_mask = 0;
@@ -3550,6 +3554,7 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms

 out_unlock:
 	mutex_unlock(&mgr->lock);
+	mutex_unlock(&mgr->payload_lock);
 	if (mstb)
 		drm_dp_mst_topology_put_mstb(mstb);
 	return ret;
-- 
2.24.1