Original issue was caused by wring detection of whether
SS modifier is being applied for render/viewport. After
recent dependency graph commit this was no longer an
issue.

But it still might have happen conflict between viewport
draw and scene_update_for_newframe() invoked from render
thread when using external engines like Cycles.

Solved by adding viewport lock while scene is being
updated and objects being exported to the render engine.

Same lock was already used for Blender Internal.