https://cs.chromium.org/chromium/src/third_party/tcmalloc/chromium/src/base/spinlock_linux-inl.h?q=futex_private_flag+&dr=CSs&l=43 currently has a static initializer to check if syscall(__NR_futex) works. That syscall has been around since linux 2.6.0 (2003) and the absl version doesn't have this check from what I can tell (https://cs.chromium.org/chromium/src/third_party/abseil-cpp/absl/synchronization/internal/waiter.cc?type=cs&q=futex_private_flag+&g=0&l=63)