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SubjectRe: [PATCH v2 03/27] drm/dp_mst: Destroy MSTBs asynchronously
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On Wed, 2019-09-25 at 14:16 -0400, Sean Paul wrote:
> 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?
>
Good question! There's only one instance where we unconditionally grab an MSTB
ref, drm_dp_mst_topology_mgr_set_mst(), and in that location we're guaranteed
to be the only one with access to that mstb until we drop &mgr->lock.
Everywhere else we use drm_dp_mst_topology_try_get_mstb(), which uses
kref_get_unless_zero() to protect against that kind of situation (and forces
the caller to check with __must_check)

> 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
> >
--
Cheers,
Lyude Paul

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