drm/i915: Rewrite some comments around RCU-deferred object free

Tvrtko noticed that the comments describing the interaction of RCU and
the deferred worker for freeing drm_i915_gem_object were a little
confusing, so attempt to bring some sense to them.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115205759.13884-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -4699,7 +4699,8 @@ static void __i915_gem_free_work(struct work_struct *work)
 		container_of(work, struct drm_i915_private, mm.free_work);
 	struct llist_node *freed;
 
-	/* All file-owned VMA should have been released by this point through
+	/*
+	 * All file-owned VMA should have been released by this point through
 	 * i915_gem_close_object(), or earlier by i915_gem_context_close().
 	 * However, the object may also be bound into the global GTT (e.g.
 	 * older GPUs without per-process support, or for direct access through
@@ -4726,10 +4727,15 @@ static void __i915_gem_free_object_rcu(struct rcu_head *head)
 		container_of(head, typeof(*obj), rcu);
 	struct drm_i915_private *i915 = to_i915(obj->base.dev);
 
-	/* We can't simply use call_rcu() from i915_gem_free_object()
-	 * as we need to block whilst unbinding, and the call_rcu
-	 * task may be called from softirq context. So we take a
-	 * detour through a worker.
+	/*
+	 * Since we require blocking on struct_mutex to unbind the freed
+	 * object from the GPU before releasing resources back to the
+	 * system, we can not do that directly from the RCU callback (which may
+	 * be a softirq context), but must instead then defer that work onto a
+	 * kthread. We use the RCU callback rather than move the freed object
+	 * directly onto the work queue so that we can mix between using the
+	 * worker and performing frees directly from subsequent allocations for
+	 * crude but effective memory throttling.
 	 */
 	if (llist_add(&obj->freed, &i915->mm.free_list))
 		queue_work(i915->wq, &i915->mm.free_work);
@@ -4745,7 +4751,8 @@ void i915_gem_free_object(struct drm_gem_object *gem_obj)
 	if (discard_backing_storage(obj))
 		obj->mm.madv = I915_MADV_DONTNEED;
 
-	/* Before we free the object, make sure any pure RCU-only
+	/*
+	 * Before we free the object, make sure any pure RCU-only
 	 * read-side critical sections are complete, e.g.
 	 * i915_gem_busy_ioctl(). For the corresponding synchronized
 	 * lookup see i915_gem_object_lookup_rcu().