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Documentation / atomic_bitops.txt




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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

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	 - 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	
66	Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
67	clear_bit_unlock() which has RELEASE semantics.
68	
69	Since a platform only has a single means of achieving atomic operations
70	the same barriers as for atomic_t are used, see atomic_t.txt.
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