=============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.