Re: [PATCH 1/2] drm/dp: look up the mstb passed into work function (v2)

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jun 22, 2015 at 02:40:43PM +1000, Dave Airlie wrote:
> From: Dave Airlie <airlied@redhat.com>
> 
> We should validate the passed in mstb under the lock
> this should stop us getting an invalid mstb here.
> 
> (first attempt with cancelling work has lockdep issues).
> Bugzilla: https://bugs.archlinux.org/task/45369
> 
> v2: update bugzilla, add some notes on why passing mst_primary
> is okay.
> 
> Signed-off-by: Dave Airlie <airlied@redhat.com>

On 2nd look I realized that pulling out mgr->lock and using the _locked
variant won't work cleanly since the _put might also take the mgr->lock. I
guess we could fix that too by also pulling the ref grab/drop for its
argument out of check_and_send like this:

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 132581ca4ad8..08c131536b2b 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -1189,6 +1189,7 @@ static void drm_dp_check_and_send_link_address(struct drm_dp_mst_topology_mgr *m
 					       struct drm_dp_mst_branch *mstb)
 {
 	struct drm_dp_mst_port *port;
+	struct drm_dp_mst_branch *mstb_child;
 
 	if (!mstb->link_address_sent) {
 		drm_dp_send_link_address(mgr, mstb);
@@ -1204,17 +1205,31 @@ static void drm_dp_check_and_send_link_address(struct drm_dp_mst_topology_mgr *m
 		if (!port->available_pbn)
 			drm_dp_send_enum_path_resources(mgr, mstb, port);
 
-		if (port->mstb)
-			drm_dp_check_and_send_link_address(mgr, port->mstb);
+		if (port->mstb) {
+			mstb_child = drm_dp_get_validated_mstb_ref(mgr,
+								   port->mstb);
+			if (mstb_child) {
+				drm_dp_check_and_send_link_address(mgr,
+								   mstb_child);
+				drm_dp_put_mst_branch_device(mstb_child);
+			}
+		}
 	}
 }
 
 static void drm_dp_mst_link_probe_work(struct work_struct *work)
 {
 	struct drm_dp_mst_topology_mgr *mgr = container_of(work, struct drm_dp_mst_topology_mgr, work);
+	struct drm_dp_mst_branch *mstb;
 
-	drm_dp_check_and_send_link_address(mgr, mgr->mst_primary);
+	mutex_lock(&mgr->lock);
+	kref_get(&mgr->mst_primary->kref);
+	mstb = mgr->mst_primary;
+	mutex_unlock(&mgr->lock);
 
+	drm_dp_check_and_send_link_address(mgr, mstb);
+
+	drm_dp_put_mst_branch_device(mstb);
 }
 
 static bool drm_dp_validate_guid(struct drm_dp_mst_topology_mgr *mgr,

Upside is that we don't need a comment, and imo non-tricky code is better
code. Or am I still missing something?
-Daniel

> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index fb65f5d..3c7e5cd 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -1198,9 +1198,14 @@ static struct drm_dp_mst_branch *drm_dp_get_mst_branch_device(struct drm_dp_mst_
>  }
>  
>  static void drm_dp_check_and_send_link_address(struct drm_dp_mst_topology_mgr *mgr,
> -					       struct drm_dp_mst_branch *mstb)
> +					       struct drm_dp_mst_branch *mstb_in)
>  {
>  	struct drm_dp_mst_port *port;
> +	struct drm_dp_mst_branch *mstb;
> +
> +	mstb = drm_dp_get_validated_mstb_ref(mgr, mstb_in);
> +	if (!mstb)
> +		return;
>  
>  	if (!mstb->link_address_sent) {
>  		drm_dp_send_link_address(mgr, mstb);
> @@ -1219,12 +1224,20 @@ static void drm_dp_check_and_send_link_address(struct drm_dp_mst_topology_mgr *m
>  		if (port->mstb)
>  			drm_dp_check_and_send_link_address(mgr, port->mstb);
>  	}
> +	drm_dp_put_mst_branch_device(mstb);
>  }
>  
>  static void drm_dp_mst_link_probe_work(struct work_struct *work)
>  {
>  	struct drm_dp_mst_topology_mgr *mgr = container_of(work, struct drm_dp_mst_topology_mgr, work);
> -
> +	/*
> +	 * Note this passes mgr->mst_primary without validation
> +	 * it could be a) valid, b) NULL, c) some value between
> +	 * since we haven't taken the lock,
> +	 * however the validation sequence in check and send
> +	 * will take the lock, and if the value isn't valid
> +	 * it will fail appropriately.
> +	 */
>  	drm_dp_check_and_send_link_address(mgr, mgr->mst_primary);
>  
>  }
> -- 
> 2.4.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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