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The spline code, especially Bezier splines, often make use of lazily evaluation and caching. In order to do that, they use mutex locks. When multi-threading, this can lead to problems. Further detail can be found in rBfcc844f8fbd0d1. To fix the deadlock, isolate the task before multi-threading when holding a lock. Differential Revision: https://developer.blender.org/D13229