must be serialized against the interrupts / soft-interrupts in which
they're manipulated, as well as protected from non-atomic 64-bit memory
loads on 32-bit platforms.
kmem_alloc() with KM_SLEEP
kmem_zalloc() with KM_SLEEP
percpu_alloc()
pserialize_create()
psref_class_create()
all of these paths include an assertion that the allocation has not failed,
so callers should not assert that again.
