[linux-2.6-block.git] / Documentation / atomic_bitops.txt


On atomic bitops.


While our bitmap_{}() functions are non-atomic, we have a number of operations
operating on single bits in a bitmap that are atomic.


API
---

The single bit operations are:

Non-RMW ops:

  test_bit()

RMW atomic operations without return value:

  {set,clear,change}_bit()
  clear_bit_unlock()

RMW atomic operations with return value:

  test_and_{set,clear,change}_bit()
  test_and_set_bit_lock()

Barriers:

  smp_mb__{before,after}_atomic()


All RMW atomic operations have a '__' prefixed variant which is non-atomic.


SEMANTICS
---------

Non-atomic ops:

In particular __clear_bit_unlock() suffers the same issue as atomic_set(),
which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.


RMW ops:

The test_and_{}_bit() operations return the original value of the bit.


ORDERING
--------

Like with atomic_t, the rule of thumb is:

 - non-RMW operations are unordered;

 - RMW operations that have no return value are unordered;

 - RMW operations that have a return value are fully ordered.

 - RMW operations that are conditional are unordered on FAILURE,
   otherwise the above rules apply. In the case of test_and_{}_bit() operations,
   if the bit in memory is unchanged by the operation then it is deemed to have
   failed.

Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
clear_bit_unlock() which has RELEASE semantics.

Since a platform only has a single means of achieving atomic operations
the same barriers as for atomic_t are used, see atomic_t.txt.
Commit	Line	Data
706eeb3e PZ	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
61e02392 WD	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
706eeb3e PZ	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.
	71