+gp_normal Make the fake writers use normal synchronous grace-period
+ primitives.
+
+gp_exp Make the fake writers use expedited synchronous grace-period
+ primitives. If both gp_normal and gp_exp are set, or
+ if neither gp_normal nor gp_exp are set, then randomly
+ choose the primitive so that about 50% are normal and
+ 50% expedited. By default, neither are set, which
+ gives best overall test coverage.
+