drm/i915: Mark the device as wedged from the beginning of set-wedged

Reduce the window of opportunity for set-wedged being called
concurrently with reset (after i915_reset() has performed the
i915_gem_unset_wedged()) by moving the set_bit(I915_WEDGED) to before we
complete the inflight requests. When i915_reset() is being blocked on a
request, such completion may allow it to start and beginning resetting
the GPU before i915_gem_set_wedged() has finished (and so before
set-wedge will have marked the device as wedged). As such,
i915_gem_init_hw() may see a wedged device even from inside
i915_reset().

References: 36703e79a982 ("drm/i915: Break modeset deadlocks on reset")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207151350.20883-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -3205,6 +3205,9 @@ void i915_gem_set_wedged(struct drm_i915_private *i915)
 			intel_engine_dump(engine, &p, "%s\n", engine->name);
 	}
 
+	set_bit(I915_WEDGED, &i915->gpu_error.flags);
+	smp_mb__after_atomic();
+
 	/*
 	 * First, stop submission to hw, but do not yet complete requests by
 	 * rolling the global seqno forward (since this would complete requests
@@ -3244,7 +3247,8 @@ void i915_gem_set_wedged(struct drm_i915_private *i915)
 	for_each_engine(engine, i915, id) {
 		unsigned long flags;
 
-		/* Mark all pending requests as complete so that any concurrent
+		/*
+		 * Mark all pending requests as complete so that any concurrent
 		 * (lockless) lookup doesn't try and wait upon the request as we
 		 * reset it.
 		 */
@@ -3254,7 +3258,6 @@ void i915_gem_set_wedged(struct drm_i915_private *i915)
 		spin_unlock_irqrestore(&engine->timeline->lock, flags);
 	}
 
-	set_bit(I915_WEDGED, &i915->gpu_error.flags);
 	wake_up_all(&i915->gpu_error.reset_queue);
 }