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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_set_bit_lock(),
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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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