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

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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

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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.