drm/i915: Hold reference to previous active fence as we queue

Take a reference to the previous exclusive fence on the i915_active, as
we wish to add an await to it in the caller (and so must prevent it from
being freed until we have completed that task).

Fixes: e3793468 ("drm/i915: Use the async worker to avoid reclaim tainting the ggtt->mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203094152.4150550-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_active.c

@@ -398,9 +398,13 @@ i915_active_set_exclusive(struct i915_active *ref, struct dma_fence *f)
 	/* We expect the caller to manage the exclusive timeline ordering */
 	GEM_BUG_ON(i915_active_is_idle(ref));
 
+	rcu_read_lock();
 	prev = __i915_active_fence_set(&ref->excl, f);
-	if (!prev)
+	if (prev)
+		prev = dma_fence_get_rcu(prev);
+	else
 		atomic_inc(&ref->count);
+	rcu_read_unlock();
 
 	return prev;
 }

drivers/gpu/drm/i915/i915_vma.c

@@ -422,10 +422,12 @@ int i915_vma_bind(struct i915_vma *vma,
 		 * execution and not content or object's backing store lifetime.
 		 */
 		prev = i915_active_set_exclusive(&vma->active, &work->base.dma);
-		if (prev)
+		if (prev) {
 			__i915_sw_fence_await_dma_fence(&work->base.chain,
 							prev,
 							&work->cb);
+			dma_fence_put(prev);
+		}
 
 		work->base.dma.error = 0; /* enable the queue_work() */