Loongson's LA664 CPU Has a Hardware Bug That Silently Loses Atomic Updates

The Lost Atomic Update on Loongson CPU

While packaging normaliz for Debian on LoongArch, Wang Miao hit an infinite loop: OpenMP's atomic counter never reached its target. Six months later, using AI to find a minimal reproducer, the root cause emerged—the LA664 core's atomic add instruction occasionally fails to be atomic when LASX vector reads interleave with barrier-less atomics on different physical cores. Loongson fixed it in two weeks with test firmware expected before October 1.