=============Atomic bitops=============While our bitmap_{}() functions are non-atomic, we have a number of operationsoperating on single bits in a bitmap that are atomic.API---The single bit operations are:Non-RMW ops:  test_bit()RMW atomic operations without return value:  {set,clear,change}_bit()  clear_bit_unlock()RMW atomic operations with return value:  test_and_{set,clear,change}_bit()  test_and_set_bit_lock()Barriers:  smp_mb__{before,after}_atomic()All RMW atomic operations have a '__' prefixed variant which is non-atomic.SEMANTICS---------Non-atomic ops:In particular __clear_bit_unlock() suffers the same issue as atomic_set(),which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.RMW ops:The test_and_{}_bit() operations return the original value of the bit.ORDERING--------Like with atomic_t, the rule of thumb is: - non-RMW operations are unordered; - RMW operations that have no return value are unordered; - RMW operations that have a return value are fully ordered. - RMW operations that are conditional are fully ordered.Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics,clear_bit_unlock() which has RELEASE semantics and test_bit_acquire which hasACQUIRE semantics.Since a platform only has a single means of achieving atomic operationsthe same barriers as for atomic_t are used, see atomic_t.txt.