drm/xe: fix xe_device_mem_access_get() races

It looks like there is at least one race here, given that the
pm_runtime_suspended() check looks to return false if we are in the
process of suspending the device (RPM_SUSPENDING vs RPM_SUSPENDED).  We
later also do xe_pm_runtime_get_if_active(), but since the device is
suspending or has now suspended, this doesn't do anything either.
Following from this we can potentially return from
xe_device_mem_access_get() with the device suspended or about to be,
leading to broken behaviour.

Attempt to fix this by always grabbing the runtime ref when our internal
ref transitions from 0 -> 1. The hard part is then dealing with the
runtime_pm callbacks also calling xe_device_mem_access_get() and
deadlocking, which the pm_runtime_suspended() check prevented.

v2:
 - ct->lock looks to be primed with fs_reclaim, so holding that and then
   allocating memory will cause lockdep to complain. Now that we
   unconditionally grab the mem_access.lock around mem_access_{get,put}, we
   need to change the ordering wrt to grabbing the ct->lock, since some of
   the runtime_pm routines can allocate memory (or at least that's what
   lockdep seems to suggest). Hopefully not a big deal.  It might be that
   there were already issues with this, just that the atomics where
   "hiding" the potential issues.
v3:
 - Use Thomas Hellström' idea with tracking the active task that is
   executing in the resume or suspend callback, in order to avoid
   recursive resume/suspend calls deadlocking on itself.
 - Split the ct->lock change.
v4:
 - Add smb_mb() around accessing the pm_callback_task for extra safety.
   (Thomas Hellström)
v5:
 - Clarify the kernel-doc for the mem_access.lock, given that it is quite
   strange in what it protects (data vs code). The real motivation is to
   aid lockdep. (Rodrigo Vivi)
v6:
 - Split out the lock change. We still want this as a lockdep aid but
   only for the xe_device_mem_access_get() path. Sticking a lock on the
   put() looks be a no-go, also the runtime_put() there is always async.
 - Now that the lock is gone move to atomics and rely on the pm code
   serialising multiple callers on the 0 -> 1 transition.
 - g2h_worker_func() looks to be the next issue, given that
   suspend-resume callbacks are using CT, so try to handle that.
v7:
 - Add xe_device_mem_access_get_if_ongoing(), and use it in
   g2h_worker_func().
v8 (Anshuman):
 - Just always grab the rpm, instead of just on the 0 -> 1 transition,
   which is a lot clearer and simplifies the code quite a bit.
v9:
 - Make sure we also adjust the CT fast-path with if-active.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/258
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Acked-by: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

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