Hi, Do we have an atomic test and set function in the kernel. I have tried const int KEY_FLAG_BIT = 1; ... static siphash_key_t ptr_secret __read_mostly; static unsigned long have_key = 0; if (test_and_set_bit(KEY_FLAG_BIT, &have_key)) get_random_bytes(&ptr_secret, sizeof(ptr_secret)); But that doesn't work. I looked in include/linux/atomic.h and thought about using static atomic_t have_key = ATOMIC_INIT(0); if (atomic_xchg(&have_key, 1) == 0) get_random_bytes(&ptr_secret, sizeof(ptr_secret)); This works. My question is; does this code LOAD the value at have_key and STORE the argument on every call? Or does it LOAD the value, check if it is the same as the argument, and STORE _only_ if it is different? (Is this whole discussion just premature optimization?) I cannot grok the macros in atomic.h, they seem circular. Here is the macro definitions in call chain order starting with atomic_xchg() #define atomic_xchg(...) \ __atomic_op_fence(atomic_xchg, __VA_ARGS__) #define __atomic_op_fence(op, args...) \ ({ \ typeof(op##_relaxed(args)) __ret; \ smp_mb__before_atomic(); \ __ret = op##_relaxed(args); \ smp_mb__after_atomic(); \ __ret; \ }) #define atomic_xchg_release(...) \ __atomic_op_release(atomic_xchg, __VA_ARGS__) #define __atomic_op_release(op, args...) \ ({ \ smp_mb__before_atomic(); \ op##_relaxed(args); \ }) #define atomic_xchg_relaxed atomic_xchg thanks, Tobin.