This commit updates the atomic fifo code to fix a consistency issue
observed on Power9 systems when builtin atomics are used. The cause
was two things: 1) a missing write memory barrier in fifo push, and 2)
a read ordering issue when reading the fifo head non-atomically. This
commit fixes both issues and appears to correct then inconsistency.
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>
8b090103e2