Re: [PATCH v2 03/27] drm/dp_mst: Destroy MSTBs asynchronously

Sean Paul
2019-09-25T18:16:47Z

On Tue, Sep 03, 2019 at 04:45:41PM -0400, Lyude Paul wrote:
> When reprobing an MST topology during resume, we have to account for the
> fact that while we were suspended it's possible that mstbs may have been
> removed from any ports in the topology. Since iterating downwards in the
> topology requires that we hold &mgr->lock, destroying MSTBs from this
> context would result in attempting to lock &mgr->lock a second time and
> deadlocking.
>
> So, fix this by first moving destruction of MSTBs into
> destroy_connector_work, then rename destroy_connector_work and friends
> to reflect that they now destroy both ports and mstbs.
>
> Changes since v1:
> * s/destroy_connector_list/destroy_port_list/
> s/connector_destroy_lock/delayed_destroy_lock/
> s/connector_destroy_work/delayed_destroy_work/
> s/drm_dp_finish_destroy_branch_device/drm_dp_delayed_destroy_mstb/
> s/drm_dp_finish_destroy_port/drm_dp_delayed_destroy_port/
> - danvet
> * Use two loops in drm_dp_delayed_destroy_work() - danvet
> * Better explain why we need to do this - danvet
> * Use cancel_work_sync() instead of flush_work() - flush_work() doesn't
> account for work requeing
>
> Cc: Juston Li <juston.li@intel.com>
> Cc: Imre Deak <imre.deak@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Harry Wentland <hwentlan@amd.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Signed-off-by: Lyude Paul <lyude@redhat.com>

Took me a while to grok this, and I'm still not 100% confident my mental model
is correct, so please bear with me while I ask silly questions :)

Now that the destroy is delayed, and the port remains in the topology, is it
possible we will underflow the topology kref by calling put_mstb multiple times?
It looks like that would result in a WARN from refcount.c, and wouldn't call the
destroy function multiple times, so that's nice :)

Similarly, is there any defense against calling get_mstb() between destroy() and
the delayed destroy worker running?

Sean

> ---
> drivers/gpu/drm/drm_dp_mst_topology.c | 162 +++++++++++++++++---------
> include/drm/drm_dp_mst_helper.h | 26 +++--
> 2 files changed, 127 insertions(+), 61 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 3054ec622506..738f260d4b15 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -1113,34 +1113,17 @@ static void drm_dp_destroy_mst_branch_device(struct kref *kref)
> struct drm_dp_mst_branch *mstb =
> container_of(kref, struct drm_dp_mst_branch, topology_kref);
> struct drm_dp_mst_topology_mgr *mgr = mstb->mgr;
> - struct drm_dp_mst_port *port, *tmp;
> - bool wake_tx = false;
>
> - mutex_lock(&mgr->lock);
> - list_for_each_entry_safe(port, tmp, &mstb->ports, next) {
> - list_del(&port->next);
> - drm_dp_mst_topology_put_port(port);
> - }
> - mutex_unlock(&mgr->lock);
> -
> - /* drop any tx slots msg */
> - mutex_lock(&mstb->mgr->qlock);
> - if (mstb->tx_slots[0]) {
> - mstb->tx_slots[0]->state = DRM_DP_SIDEBAND_TX_TIMEOUT;
> - mstb->tx_slots[0] = NULL;
> - wake_tx = true;
> - }
> - if (mstb->tx_slots[1]) {
> - mstb->tx_slots[1]->state = DRM_DP_SIDEBAND_TX_TIMEOUT;
> - mstb->tx_slots[1] = NULL;
> - wake_tx = true;
> - }
> - mutex_unlock(&mstb->mgr->qlock);
> + INIT_LIST_HEAD(&mstb->destroy_next);
>
> - if (wake_tx)
> - wake_up_all(&mstb->mgr->tx_waitq);
> -
> - drm_dp_mst_put_mstb_malloc(mstb);
> + /*
> + * This can get called under mgr->mutex, so we need to perform the
> + * actual destruction of the mstb in another worker
> + */
> + mutex_lock(&mgr->delayed_destroy_lock);
> + list_add(&mstb->destroy_next, &mgr->destroy_branch_device_list);
> + mutex_unlock(&mgr->delayed_destroy_lock);
> + schedule_work(&mgr->delayed_destroy_work);
> }
>
> /**
> @@ -1255,10 +1238,10 @@ static void drm_dp_destroy_port(struct kref *kref)
> * we might be holding the mode_config.mutex
> * from an EDID retrieval */
>
> - mutex_lock(&mgr->destroy_connector_lock);
> - list_add(&port->next, &mgr->destroy_connector_list);
> - mutex_unlock(&mgr->destroy_connector_lock);
> - schedule_work(&mgr->destroy_connector_work);
> + mutex_lock(&mgr->delayed_destroy_lock);
> + list_add(&port->next, &mgr->destroy_port_list);
> + mutex_unlock(&mgr->delayed_destroy_lock);
> + schedule_work(&mgr->delayed_destroy_work);
> return;
> }
> /* no need to clean up vcpi
> @@ -2792,7 +2775,7 @@ void drm_dp_mst_topology_mgr_suspend(struct drm_dp_mst_topology_mgr *mgr)
> DP_MST_EN | DP_UPSTREAM_IS_SRC);
> mutex_unlock(&mgr->lock);
> flush_work(&mgr->work);
> - flush_work(&mgr->destroy_connector_work);
> + flush_work(&mgr->delayed_destroy_work);
> }
> EXPORT_SYMBOL(drm_dp_mst_topology_mgr_suspend);
>
> @@ -3740,34 +3723,104 @@ static void drm_dp_tx_work(struct work_struct *work)
> mutex_unlock(&mgr->qlock);
> }
>
> -static void drm_dp_destroy_connector_work(struct work_struct *work)
> +static inline void
> +drm_dp_delayed_destroy_port(struct drm_dp_mst_port *port)
> {
> - struct drm_dp_mst_topology_mgr *mgr = container_of(work, struct drm_dp_mst_topology_mgr, destroy_connector_work);
> - struct drm_dp_mst_port *port;
> - bool send_hotplug = false;
> + port->mgr->cbs->destroy_connector(port->mgr, port->connector);
> +
> + drm_dp_port_teardown_pdt(port, port->pdt);
> + port->pdt = DP_PEER_DEVICE_NONE;
> +
> + drm_dp_mst_put_port_malloc(port);
> +}
> +
> +static inline void
> +drm_dp_delayed_destroy_mstb(struct drm_dp_mst_branch *mstb)
> +{
> + struct drm_dp_mst_topology_mgr *mgr = mstb->mgr;
> + struct drm_dp_mst_port *port, *tmp;
> + bool wake_tx = false;
> +
> + mutex_lock(&mgr->lock);
> + list_for_each_entry_safe(port, tmp, &mstb->ports, next) {
> + list_del(&port->next);
> + drm_dp_mst_topology_put_port(port);
> + }
> + mutex_unlock(&mgr->lock);
> +
> + /* drop any tx slots msg */
> + mutex_lock(&mstb->mgr->qlock);
> + if (mstb->tx_slots[0]) {
> + mstb->tx_slots[0]->state = DRM_DP_SIDEBAND_TX_TIMEOUT;
> + mstb->tx_slots[0] = NULL;
> + wake_tx = true;
> + }
> + if (mstb->tx_slots[1]) {
> + mstb->tx_slots[1]->state = DRM_DP_SIDEBAND_TX_TIMEOUT;
> + mstb->tx_slots[1] = NULL;
> + wake_tx = true;
> + }
> + mutex_unlock(&mstb->mgr->qlock);
> +
> + if (wake_tx)
> + wake_up_all(&mstb->mgr->tx_waitq);
> +
> + drm_dp_mst_put_mstb_malloc(mstb);
> +}
> +
> +static void drm_dp_delayed_destroy_work(struct work_struct *work)
> +{
> + struct drm_dp_mst_topology_mgr *mgr =
> + container_of(work, struct drm_dp_mst_topology_mgr,
> + delayed_destroy_work);
> + bool send_hotplug = false, go_again;
> +
> /*
> * Not a regular list traverse as we have to drop the destroy
> - * connector lock before destroying the connector, to avoid AB->BA
> + * connector lock before destroying the mstb/port, to avoid AB->BA
> * ordering between this lock and the config mutex.
> */
> - for (;;) {
> - mutex_lock(&mgr->destroy_connector_lock);
> - port = list_first_entry_or_null(&mgr->destroy_connector_list, struct drm_dp_mst_port, next);
> - if (!port) {
> - mutex_unlock(&mgr->destroy_connector_lock);
> - break;
> + do {
> + go_again = false;
> +
> + for (;;) {
> + struct drm_dp_mst_branch *mstb;
> +
> + mutex_lock(&mgr->delayed_destroy_lock);
> + mstb = list_first_entry_or_null(&mgr->destroy_branch_device_list,
> + struct drm_dp_mst_branch,
> + destroy_next);
> + if (mstb)
> + list_del(&mstb->destroy_next);
> + mutex_unlock(&mgr->delayed_destroy_lock);
> +
> + if (!mstb)
> + break;
> +
> + drm_dp_delayed_destroy_mstb(mstb);
> + go_again = true;
> }
> - list_del(&port->next);
> - mutex_unlock(&mgr->destroy_connector_lock);
>
> - mgr->cbs->destroy_connector(mgr, port->connector);
> + for (;;) {
> + struct drm_dp_mst_port *port;
>
> - drm_dp_port_teardown_pdt(port, port->pdt);
> - port->pdt = DP_PEER_DEVICE_NONE;
> + mutex_lock(&mgr->delayed_destroy_lock);
> + port = list_first_entry_or_null(&mgr->destroy_port_list,
> + struct drm_dp_mst_port,
> + next);
> + if (port)
> + list_del(&port->next);
> + mutex_unlock(&mgr->delayed_destroy_lock);
> +
> + if (!port)
> + break;
> +
> + drm_dp_delayed_destroy_port(port);
> + send_hotplug = true;
> + go_again = true;
> + }
> + } while (go_again);
>
> - drm_dp_mst_put_port_malloc(port);
> - send_hotplug = true;
> - }
> if (send_hotplug)
> drm_kms_helper_hotplug_event(mgr->dev);
> }
> @@ -3957,12 +4010,13 @@ int drm_dp_mst_topology_mgr_init(struct drm_dp_mst_topology_mgr *mgr,
> mutex_init(&mgr->lock);
> mutex_init(&mgr->qlock);
> mutex_init(&mgr->payload_lock);
> - mutex_init(&mgr->destroy_connector_lock);
> + mutex_init(&mgr->delayed_destroy_lock);
> INIT_LIST_HEAD(&mgr->tx_msg_downq);
> - INIT_LIST_HEAD(&mgr->destroy_connector_list);
> + INIT_LIST_HEAD(&mgr->destroy_port_list);
> + INIT_LIST_HEAD(&mgr->destroy_branch_device_list);
> INIT_WORK(&mgr->work, drm_dp_mst_link_probe_work);
> INIT_WORK(&mgr->tx_work, drm_dp_tx_work);
> - INIT_WORK(&mgr->destroy_connector_work, drm_dp_destroy_connector_work);
> + INIT_WORK(&mgr->delayed_destroy_work, drm_dp_delayed_destroy_work);
> init_waitqueue_head(&mgr->tx_waitq);
> mgr->dev = dev;
> mgr->aux = aux;
> @@ -4005,7 +4059,7 @@ void drm_dp_mst_topology_mgr_destroy(struct drm_dp_mst_topology_mgr *mgr)
> {
> drm_dp_mst_topology_mgr_set_mst(mgr, false);
> flush_work(&mgr->work);
> - flush_work(&mgr->destroy_connector_work);
> + cancel_work_sync(&mgr->delayed_destroy_work);
> mutex_lock(&mgr->payload_lock);
> kfree(mgr->payloads);
> mgr->payloads = NULL;
> diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
> index fc349204a71b..4a4507fe928d 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -143,6 +143,12 @@ struct drm_dp_mst_branch {
> */
> struct kref malloc_kref;
>
> + /**
> + * @destroy_next: linked-list entry used by
> + * drm_dp_delayed_destroy_work()
> + */
> + struct list_head destroy_next;
> +
> u8 rad[8];
> u8 lct;
> int num_ports;
> @@ -575,18 +581,24 @@ struct drm_dp_mst_topology_mgr {
> struct work_struct tx_work;
>
> /**
> - * @destroy_connector_list: List of to be destroyed connectors.
> + * @destroy_port_list: List of to be destroyed connectors.
> + */
> + struct list_head destroy_port_list;
> + /**
> + * @destroy_branch_device_list: List of to be destroyed branch
> + * devices.
> */
> - struct list_head destroy_connector_list;
> + struct list_head destroy_branch_device_list;
> /**
> - * @destroy_connector_lock: Protects @connector_list.
> + * @delayed_destroy_lock: Protects @destroy_port_list and
> + * @destroy_branch_device_list.
> */
> - struct mutex destroy_connector_lock;
> + struct mutex delayed_destroy_lock;
> /**
> - * @destroy_connector_work: Work item to destroy connectors. Needed to
> - * avoid locking inversion.
> + * @delayed_destroy_work: Work item to destroy MST port and branch
> + * devices, needed to avoid locking inversion.
> */
> - struct work_struct destroy_connector_work;
> + struct work_struct delayed_destroy_work;
> };
>
> int drm_dp_mst_topology_mgr_init(struct drm_dp_mst_topology_mgr *mgr,
> --
> 2.21.0
>

--
Sean Paul, Software Engineer, Google / Chromium OS