drm/i915: Wrap engine->schedule in RCU locks for set-wedge protection

Similar to the staging around handling of engine->submit_request, we
need to stop adding to the execlists->queue prior to calling
engine->cancel_requests. cancel_requests will move requests from the
queue onto the timeline, so if we add a request onto the queue after that
point, it will be lost.

Fixes: af7a8ffad9c5 ("drm/i915: Use rcu instead of stop_machine in set_wedged")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180307134226.25492-5-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -479,10 +479,11 @@ static void __fence_set_priority(struct dma_fence *fence, int prio)
 
 	rq = to_request(fence);
 	engine = rq->engine;
-	if (!engine->schedule)
-		return;
 
-	engine->schedule(rq, prio);
+	rcu_read_lock();
+	if (engine->schedule)
+		engine->schedule(rq, prio);
+	rcu_read_unlock();
 }
 
 static void fence_set_priority(struct dma_fence *fence, int prio)
@@ -3222,8 +3223,11 @@ void i915_gem_set_wedged(struct drm_i915_private *i915)
 	 */
 	for_each_engine(engine, i915, id) {
 		i915_gem_reset_prepare_engine(engine);
+
 		engine->submit_request = nop_submit_request;
+		engine->schedule = NULL;
 	}
+	i915->caps.scheduler = 0;
 
 	/*
 	 * Make sure no one is running the old callback before we proceed with
@@ -3241,11 +3245,8 @@ void i915_gem_set_wedged(struct drm_i915_private *i915)
 		 * start to complete all requests.
 		 */
 		engine->submit_request = nop_complete_submit_request;
-		engine->schedule = NULL;
 	}
 
-	i915->caps.scheduler = 0;
-
 	/*
 	 * Make sure no request can slip through without getting completed by
 	 * either this call here to intel_engine_init_global_seqno, or the one

drivers/gpu/drm/i915/i915_request.c

@@ -1081,8 +1081,10 @@ void __i915_request_add(struct i915_request *request, bool flush_caches)
 	 * decide whether to preempt the entire chain so that it is ready to
 	 * run at the earliest possible convenience.
 	 */
+	rcu_read_lock();
 	if (engine->schedule)
 		engine->schedule(request, request->ctx->priority);
+	rcu_read_unlock();
 
 	local_bh_disable();
 	i915_sw_fence_commit(&request->submit);