Turn off smoothing or

Raise shadow min bias to .500 or above for RT shadows or

apply an additional level of subdivision or

Use depth mapped shadows.

It's polygonal self shadowing brought about by reyes aspect of Firefly, PITA. Turn off RT shadows and do an area render of a spot that is heavy with artifacts... you'll see that they're no longer there.

Edit_ sometimes turning off smoothing doesn't work as well as expected.

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