1
2On atomic 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_{}_bit() operations,
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