drm/dp_mst: Add probe_lock

drm/dp_mst: Add probe_lock

Currently, MST lacks locking in a lot of places that really should have
some sort of locking. Hotplugging and link address code paths are some
of the offenders here, as there is actually nothing preventing us from
running a link address probe while at the same time handling a
connection status update request - something that's likely always been
possible but never seen in the wild because hotplugging has been broken
for ages now (with the exception of amdgpu, for reasons I don't think
are worth digging into very far).

Note: I'm going to start using the term "in-memory topology layout" here
to refer to drm_dp_mst_port->mstb and drm_dp_mst_branch->ports.

Locking in these places is a little tougher then it looks though.
Generally we protect anything having to do with the in-memory topology
layout under &mgr->lock. But this becomes nearly impossible to do from
the context of link address probes due to the fact that &mgr->lock is
usually grabbed under random various modesetting locks, meaning that
there's no way we can just invert the &mgr->lock order and keep it
locked throughout the whole process of updating the topology.

Luckily there are only two workers which can modify the in-memory
topology layout: drm_dp_mst_up_req_work() and
drm_dp_mst_link_probe_work(), meaning as long as we prevent these two
workers from traveling the topology layout in parallel with the intent
of updating it we don't need to worry about grabbing &mgr->lock in these
workers for reads. We only need to grab &mgr->lock in these workers for
writes, so that readers outside these two workers are still protected
from the topology layout changing beneath them.

So, add the new &mgr->probe_lock and use it in both
drm_dp_mst_link_probe_work() and drm_dp_mst_up_req_work(). Additionally,
add some more detailed explanations for how this locking is intended to
work to drm_dp_mst_port->mstb and drm_dp_mst_branch->ports.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Sean Paul <sean@poorly.run>
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.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022023641.8026-6-lyude@redhat.com