1============= 2Atomic bitops 3============= 4 5While our bitmap_{}() functions are non-atomic, we have a number of operations 6operating on single bits in a bitmap that are atomic. 7 8 9API 10--- 11 12The single bit operations are: 13 14Non-RMW ops: 15 16 test_bit() 17 18RMW atomic operations without return value: 19 20 {set,clear,change}_bit() 21 clear_bit_unlock() 22 23RMW atomic operations with return value: 24 25 test_and_{set,clear,change}_bit() 26 test_and_set_bit_lock() 27 28Barriers: 29 30 smp_mb__{before,after}_atomic() 31 32 33All RMW atomic operations have a '__' prefixed variant which is non-atomic. 34 35 36SEMANTICS 37--------- 38 39Non-atomic ops: 40 41In particular __clear_bit_unlock() suffers the same issue as atomic_set(), 42which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt. 43 44 45RMW ops: 46 47The test_and_{}_bit() operations return the original value of the bit. 48 49 50ORDERING 51-------- 52 53Like with atomic_t, the rule of thumb is: 54 55 - non-RMW operations are unordered; 56 57 - RMW operations that have no return value are unordered; 58 59 - RMW operations that have a return value are fully ordered. 60 61 - RMW operations that are conditional are unordered on FAILURE, 62 otherwise the above rules apply. In the case of test_and_set_bit_lock(), 63 if the bit in memory is unchanged by the operation then it is deemed to have 64 failed. 65 66Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and 67clear_bit_unlock() which has RELEASE semantics. 68 69Since a platform only has a single means of achieving atomic operations 70the same barriers as for atomic_t are used, see atomic_t.txt. 71 72