drm/i915/gt: Wait for RCUs frees before asserting idle on unload

During driver unload, we have many asserts that we have released our
bookkeeping structs and are idle. In some cases, these struct are
protected by RCU and we do not release them until after an RCU grace
period.
Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 130a95e9 ("drm/i915/gem: Consolidate ctx->engines[] release")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200312115307.16460-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -903,6 +903,7 @@ void i915_gem_init__contexts(struct drm_i915_private *i915)
 void i915_gem_driver_release__contexts(struct drm_i915_private *i915)
 {
 	flush_work(&i915->gem.contexts.free_work);
+	rcu_barrier(); /* and flush the left over RCU frees */
 }
 
 static int gem_context_register(struct i915_gem_context *ctx,

drivers/gpu/drm/i915/gt/intel_gt.c

@@ -667,6 +667,9 @@ void intel_gt_driver_release(struct intel_gt *gt)
 
 void intel_gt_driver_late_release(struct intel_gt *gt)
 {
+	/* We need to wait for inflight RCU frees to release their grip */
+	rcu_barrier();
+
 	intel_uc_driver_late_release(&gt->uc);
 	intel_gt_fini_requests(gt);
 	intel_gt_fini_reset(gt);