============= Atomic bitops ============= While our bitmap_{}() functions are non-atomic, we have a number of operations operating 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 unordered on FAILURE, otherwise the above rules apply. In the case of test_and_{}_bit() operations, if the bit in memory is unchanged by the operation then it is deemed to have failed. Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and clear_bit_unlock() which has RELEASE semantics. Since a platform only has a single means of achieving atomic operations the same barriers as for atomic_t are used, see atomic_t.txt.