ARMCC documentation states that the __dsb etc intrinsics act as
optimisation barriers. Even though that's a bit woolly about the exact
equivalent barrier intrinsic, take its word that it's doing the right
thing.
It seems safe to assume that it is, because the __schedule_barrier()
intrinsics here are not actually sufficient for DSB and DMB. They need a
__memory_barrier(). If no-one has seen any problems, then presumably
they already include one.
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