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

From: Daniel Vetter
Date: Tue Oct 08 2019 - 05:45:51 EST

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On Fri, Sep 27, 2019 at 09:31:07AM -0400, Sean Paul wrote:
> On Wed, Sep 25, 2019 at 04:08:22PM -0400, Lyude Paul wrote:
> > 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@xxxxxxxxx>
> > > > Cc: Imre Deak <imre.deak@xxxxxxxxx>
> > > > Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > > > Cc: Harry Wentland <hwentlan@xxxxxxx>
> > > > Cc: Daniel Vetter <daniel@xxxxxxxx>
> > > > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > >
> > > 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)

Imo would be good to add this to the commit message when merging as a Q&A,
just for the record. At least I like to do that with the
silly-but-no-so-silly questions that come up in review.
-Daniel

>
> Awesome, thanks for the breakdown!
>
>
> Reviewed-by: Sean Paul <sean@xxxxxxxxxx>
>
>
> >
> > > 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
> >
>
> --
> Sean Paul, Software Engineer, Google / Chromium OS

--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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