drm/i915/execlists: Ensure the tasklet is decoupled upon shutdown

As we only cancel the timers asynchronously, they may
still be running on another CPU as we shutdown, raising one last
softirq. So be safe and make sure the tasklet is flushed before
destroying the engine's memory.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129172542.1222810-1-chris@chris-wilson.co.uk

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

@@ -3777,8 +3777,18 @@ void intel_execlists_set_default_submission(struct intel_engine_cs *engine)
 		engine->flags |= I915_ENGINE_HAS_RELATIVE_MMIO;
 }
 
+static void execlists_shutdown(struct intel_engine_cs *engine)
+{
+	/* Synchronise with residual timers and any softirq they raise */
+	del_timer_sync(&engine->execlists.timer);
+	del_timer_sync(&engine->execlists.preempt);
+	tasklet_kill(&engine->execlists.tasklet);
+}
+
 static void execlists_destroy(struct intel_engine_cs *engine)
 {
+	execlists_shutdown(engine);
+
 	intel_engine_cleanup_common(engine);
 	lrc_destroy_wa_ctx(engine);
 	kfree(engine);