drm/i915: Order two completing nop_submit_request

If two nop's (requests in-flight following a wedged device) complete at
the same time, the global_seqno value written to the HWSP is undefined
as the two threads are not serialized.

v2: Use irqsafe spinlock. We expect the callback may be called from
inside another irq spinlock, so we can't unconditionally restore irqs.

Fixes: ce1135c7 ("drm/i915: Complete requests in nop_submit_request")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171006115617.18432-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -3014,10 +3014,15 @@ void i915_gem_reset_finish(struct drm_i915_private *dev_priv)
 
 static void nop_submit_request(struct drm_i915_gem_request *request)
 {
+	unsigned long flags;
+
 	GEM_BUG_ON(!i915_terminally_wedged(&request->i915->gpu_error));
 	dma_fence_set_error(&request->fence, -EIO);
-	i915_gem_request_submit(request);
+
+	spin_lock_irqsave(&request->engine->timeline->lock, flags);
+	__i915_gem_request_submit(request);
 	intel_engine_init_global_seqno(request->engine, request->global_seqno);
+	spin_unlock_irqrestore(&request->engine->timeline->lock, flags);
 }
 
 static void engine_set_wedged(struct intel_engine_cs *engine)