Why do atomic<int>::wait use timed futex?

MBkkt · September 18, 2022, 3:13am

llvm/llvm-project/blob/main/libcxx/src/atomic.cpp#L42


      
          
          
#endif
          
          
_LIBCPP_BEGIN_NAMESPACE_STD
          
          
#ifdef __linux__
          
          
static void __libcpp_platform_wait_on_address(__cxx_atomic_contention_t const volatile* __ptr,
                                                        __cxx_contention_t __val)
          {
              static constexpr timespec __timeout = { 2, 0 };
              syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
          }
          
          
static void __libcpp_platform_wake_by_address(__cxx_atomic_contention_t const volatile* __ptr,
                                                        bool __notify_one)
          {
              syscall(SYS_futex, __ptr, FUTEX_WAKE_PRIVATE, __notify_one ? 1 : INT_MAX, 0, 0, 0);
          }
          
          
#elif defined(__APPLE__) && defined(_LIBCPP_USE_ULOCK)

I don’t think something like wakeup every two seconds is really bad, but in the same time it doesn’t looks good.

At the same time, on macOS used not timed wait