Since commit 0709b7ee, spinlock primitives include a compiler barrier
so it is no longer necessary to access either spinlocks or the memory
they protect through pointer-to-volatile.  Like earlier commits
e93b6298, d53e3d5f, 430008b5, 8f6bb851, df4077cd.

Author: Thomas Munro
Discussion: https://postgr.es/m/CAEepm=204T37SxcHo4=xw5btho9jQ-=ZYYrVdcKyz82XYzMoqg@mail.gmail.com