Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net
[linux-2.6-block.git] / Documentation / atomic_bitops.txt
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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:
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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.
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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
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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
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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