=============
  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.