1 =============
2 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 - RMW operations that are conditional are fully ordered.
62
63 Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics,
64 clear_bit_unlock() which has RELEASE semantics and test_bit_acquire which has
65 ACQUIRE semantics.
66
67 Since a platform only has a single means of achieving atomic operations
68 the same barriers as for atomic_t are used, see atomic_t.txt.
69
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