Documentation/atomic_bitops.txt

   1
   2 On atomic bitops.
   3
   4
   5 While our bitmap_{}() functions are non-atomic, we have a number of operations
   6 operating on single bits in a bitmap that are atomic.
   7
   8
   9 API
  10 ---
  11
  12 The single bit operations are:
  13
  14 Non-RMW ops:
  15
  16   test_bit()
  17
  18 RMW atomic operations without return value:
  19
  20   {set,clear,change}_bit()
  21   clear_bit_unlock()
  22
  23 RMW atomic operations with return value:
  24
  25   test_and_{set,clear,change}_bit()
  26   test_and_set_bit_lock()
  27
  28 Barriers:
  29
  30   smp_mb__{before,after}_atomic()
  31
  32
  33 All RMW atomic operations have a '__' prefixed variant which is non-atomic.
  34
  35
  36 SEMANTICS
  37 ---------
  38
  39 Non-atomic ops:
  40
  41 In particular __clear_bit_unlock() suffers the same issue as atomic_set(),
  42 which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.
  43
  44
  45 RMW ops:
  46
  47 The test_and_{}_bit() operations return the original value of the bit.
  48
  49
  50 ORDERING
  51 --------
  52
  53 Like 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 Except for test_and_set_bit_lock() which has ACQUIRE semantics and
  62 clear_bit_unlock() which has RELEASE semantics.
  63
  64 Since a platform only has a single means of achieving atomic operations
  65 the same barriers as for atomic_t are used, see atomic_t.txt.
  66