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On atomic types (atomic_t atomic64_t and atomic_long_t).

The atomic type provides an interface to the architecture's means of atomic
RMW operations between CPUs (atomic operations on MMIO are not supported and
can lead to fatal traps on some platforms).

API
---

The 'full' API consists of (atomic64_ and atomic_long_ prefixes omitted for
brevity):

Non-RMW ops:

  atomic_read(), atomic_set()
  atomic_read_acquire(), atomic_set_release()


RMW atomic operations:

Arithmetic:

  atomic_{add,sub,inc,dec}()
  atomic_{add,sub,inc,dec}_return{,_relaxed,_acquire,_release}()
  atomic_fetch_{add,sub,inc,dec}{,_relaxed,_acquire,_release}()


Bitwise:

  atomic_{and,or,xor,andnot}()
  atomic_fetch_{and,or,xor,andnot}{,_relaxed,_acquire,_release}()


Swap:

  atomic_xchg{,_relaxed,_acquire,_release}()
  atomic_cmpxchg{,_relaxed,_acquire,_release}()
  atomic_try_cmpxchg{,_relaxed,_acquire,_release}()


Reference count (but please see refcount_t):

  atomic_add_unless(), atomic_inc_not_zero()
  atomic_sub_and_test(), atomic_dec_and_test()


Misc:

  atomic_inc_and_test(), atomic_add_negative()
  atomic_dec_unless_positive(), atomic_inc_unless_negative()


Barriers:

  smp_mb__{before,after}_atomic()



SEMANTICS
---------

Non-RMW ops:

The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
smp_store_release() respectively.

The one detail to this is that atomic_set{}() should be observable to the RMW
ops. That is:

  C atomic-set

  {
    atomic_set(v, 1);
  }

  P1(atomic_t *v)
  {
    atomic_add_unless(v, 1, 0);
  }

  P2(atomic_t *v)
  {
    atomic_set(v, 0);
  }

  exists
  (v=2)

In this case we would expect the atomic_set() from CPU1 to either happen
before the atomic_add_unless(), in which case that latter one would no-op, or
_after_ in which case we'd overwrite its result. In no case is "2" a valid
outcome.

This is typically true on 'normal' platforms, where a regular competing STORE
will invalidate a LL/SC or fail a CMPXCHG.

The obvious case where this is not so is when we need to implement atomic ops
with a lock:

  CPU0						CPU1

  atomic_add_unless(v, 1, 0);
    lock();
    ret = READ_ONCE(v->counter); // == 1
						atomic_set(v, 0);
    if (ret != u)				  WRITE_ONCE(v->counter, 0);
      WRITE_ONCE(v->counter, ret + 1);
    unlock();

the typical solution is to then implement atomic_set{}() with atomic_xchg().


RMW ops:

These come in various forms:

 - plain operations without return value: atomic_{}()

 - operations which return the modified value: atomic_{}_return()

   these are limited to the arithmetic operations because those are
   reversible. Bitops are irreversible and therefore the modified value
   is of dubious utility.

 - operations which return the original value: atomic_fetch_{}()

 - swap operations: xchg(), cmpxchg() and try_cmpxchg()

 - misc; the special purpose operations that are commonly used and would,
   given the interface, normally be implemented using (try_)cmpxchg loops but
   are time critical and can, (typically) on LL/SC architectures, be more
   efficiently implemented.

All these operations are SMP atomic; that is, the operations (for a single
atomic variable) can be fully ordered and no intermediate state is lost or
visible.


ORDERING  (go read memory-barriers.txt first)
--------

The rule of thumb:

 - 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.

Except of course when an operation has an explicit ordering like:

 {}_relaxed: unordered
 {}_acquire: the R of the RMW (or atomic_read) is an ACQUIRE
 {}_release: the W of the RMW (or atomic_set)  is a  RELEASE

Where 'unordered' is against other memory locations. Address dependencies are
not defeated.

Fully ordered primitives are ordered against everything prior and everything
subsequent. Therefore a fully ordered primitive is like having an smp_mb()
before and an smp_mb() after the primitive.


The barriers:

  smp_mb__{before,after}_atomic()

only apply to the RMW ops and can be used to augment/upgrade the ordering
inherent to the used atomic op. These barriers provide a full smp_mb().

These helper barriers exist because architectures have varying implicit
ordering on their SMP atomic primitives. For example our TSO architectures
provide full ordered atomics and these barriers are no-ops.

Thus:

  atomic_fetch_add();

is equivalent to:

  smp_mb__before_atomic();
  atomic_fetch_add_relaxed();
  smp_mb__after_atomic();

However the atomic_fetch_add() might be implemented more efficiently.

Further, while something like:

  smp_mb__before_atomic();
  atomic_dec(&X);

is a 'typical' RELEASE pattern, the barrier is strictly stronger than
a RELEASE. Similarly for something like: