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.


The single bit operations are:

Non-RMW ops:


RMW atomic operations without return value:


RMW atomic operations with return value:




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


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.


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

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.