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71 lines
1.5 KiB
71 lines
1.5 KiB
============= |
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Atomic bitops |
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============= |
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While our bitmap_{}() functions are non-atomic, we have a number of operations |
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operating on single bits in a bitmap that are atomic. |
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API |
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--- |
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The single bit operations are: |
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Non-RMW ops: |
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test_bit() |
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RMW atomic operations without return value: |
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{set,clear,change}_bit() |
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clear_bit_unlock() |
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RMW atomic operations with return value: |
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test_and_{set,clear,change}_bit() |
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test_and_set_bit_lock() |
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Barriers: |
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smp_mb__{before,after}_atomic() |
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All RMW atomic operations have a '__' prefixed variant which is non-atomic. |
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SEMANTICS |
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--------- |
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Non-atomic ops: |
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In particular __clear_bit_unlock() suffers the same issue as atomic_set(), |
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which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt. |
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RMW ops: |
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The test_and_{}_bit() operations return the original value of the bit. |
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ORDERING |
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-------- |
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Like with atomic_t, the rule of thumb is: |
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- non-RMW operations are unordered; |
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- RMW operations that have no return value are unordered; |
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- RMW operations that have a return value are fully ordered. |
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- RMW operations that are conditional are unordered on FAILURE, |
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otherwise the above rules apply. In the case of test_and_{}_bit() operations, |
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if the bit in memory is unchanged by the operation then it is deemed to have |
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failed. |
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Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and |
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clear_bit_unlock() which has RELEASE semantics. |
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Since a platform only has a single means of achieving atomic operations |
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the same barriers as for atomic_t are used, see atomic_t.txt. |
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