atomic_t.txt

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()


TYPES (signed vs unsigned)
-----

While atomic_t, atomic_long_t and atomic64_t use int, long and s64
respectively (for hysterical raisins), the kernel uses -fno-strict-overflow
(which implies -fwrapv) and defines signed overflow to behave like
2s-complement.

Therefore, an explicitly unsigned variant of the atomic ops is strictly
unnecessary and we can simply cast, there is no UB.

There was a bug in UBSAN prior to GCC-8 that would generate UB warnings for
signed types.

With this we also conform to the C/C++ _Atomic behaviour and things like
P1236R1.


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. Therefore, if you find yourself only using
the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
and are doing it wrong.

A note for the implementation of atomic_set{}() is that it must not break the
atomicity of the RMW ops. That is:

  C Atomic-RMW-ops-are-atomic-WRT-atomic_set

  {
    atomic_t v = ATOMIC_INIT(1);
  }

  P0(atomic_t *v)
  {
    (void)atomic_add_unless(v, 1, 0);
  }

  P1(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 atomic ops and can be used to augment/upgrade the
ordering inherent to the op. These barriers act almost like a full smp_mb():
smp_mb__before_atomic() orders all earlier accesses against the RMW op
itself and all accesses following it, and smp_mb__after_atomic() orders all
later accesses against the RMW op and all accesses preceding it. However,
accesses between the smp_mb__{before,after}_atomic() and the RMW op are not
ordered, so it is advisable to place the barrier right next to the RMW atomic
op whenever possible.

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.

NOTE: when the atomic RmW ops are fully ordered, they should also imply a
compiler barrier.

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 because it orders preceding instructions against both the read
and write parts of the atomic_dec(), and against all following instructions
as well. Similarly, something like:

  atomic_inc(&X);
  smp_mb__after_atomic();

is an ACQUIRE pattern (though very much not typical), but again the barrier is
strictly stronger than ACQUIRE. As illustrated:

  C Atomic-RMW+mb__after_atomic-is-stronger-than-acquire

  {
  }

  P0(int *x, atomic_t *y)
  {
    r0 = READ_ONCE(*x);
    smp_rmb();
    r1 = atomic_read(y);
  }

  P1(int *x, atomic_t *y)
  {
    atomic_inc(y);
    smp_mb__after_atomic();
    WRITE_ONCE(*x, 1);
  }

  exists
  (0:r0=1 /\ 0:r1=0)

This should not happen; but a hypothetical atomic_inc_acquire() --
(void)atomic_fetch_inc_acquire() for instance -- would allow the outcome,
because it would not order the W part of the RMW against the following
WRITE_ONCE.  Thus:

  P0			P1

			t = LL.acq *y (0)
			t++;
			*x = 1;
  r0 = *x (1)
  RMB
  r1 = *y (0)
			SC *y, t;

is allowed.


CMPXCHG vs TRY_CMPXCHG
----------------------

  int atomic_cmpxchg(atomic_t *ptr, int old, int new);
  bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new);

Both provide the same functionality, but try_cmpxchg() can lead to more
compact code. The functions relate like:

  bool atomic_try_cmpxchg(atomic_t *ptr, int *oldp, int new)
  {
    int ret, old = *oldp;
    ret = atomic_cmpxchg(ptr, old, new);
    if (ret != old)
      *oldp = ret;
    return ret == old;
  }

and:

  int atomic_cmpxchg(atomic_t *ptr, int old, int new)
  {
    (void)atomic_try_cmpxchg(ptr, &old, new);
    return old;
  }

Usage:

  old = atomic_read(&v);			old = atomic_read(&v);
  for (;;) {					do {
    new = func(old);				  new = func(old);
    tmp = atomic_cmpxchg(&v, old, new);		} while (!atomic_try_cmpxchg(&v, &old, new));
    if (tmp == old)
      break;
    old = tmp;
  }

NB. try_cmpxchg() also generates better code on some platforms (notably x86)
where the function more closely matches the hardware instruction.


FORWARD PROGRESS
----------------

In general strong forward progress is expected of all unconditional atomic
operations -- those in the Arithmetic and Bitwise classes and xchg(). However
a fair amount of code also requires forward progress from the conditional
atomic operations.

Specifically 'simple' cmpxchg() loops are expected to not starve one another
indefinitely. However, this is not evident on LL/SC architectures, because
while an LL/SC architecure 'can/should/must' provide forward progress
guarantees between competing LL/SC sections, such a guarantee does not
transfer to cmpxchg() implemented using LL/SC. Consider:

  old = atomic_read(&v);
  do {
    new = func(old);
  } while (!atomic_try_cmpxchg(&v, &old, new));

which on LL/SC becomes something like:

  old = atomic_read(&v);
  do {
    new = func(old);
  } while (!({
    volatile asm ("1: LL  %[oldval], %[v]\n"
                  "   CMP %[oldval], %[old]\n"
                  "   BNE 2f\n"
                  "   SC  %[new], %[v]\n"
                  "   BNE 1b\n"
                  "2:\n"
                  : [oldval] "=&r" (oldval), [v] "m" (v)
		  : [old] "r" (old), [new] "r" (new)
                  : "memory");
    success = (oldval == old);
    if (!success)
      old = oldval;
    success; }));

However, even the forward branch from the failed compare can cause the LL/SC
to fail on some architectures, let alone whatever the compiler makes of the C
loop body. As a result there is no guarantee what so ever the cacheline
containing @v will stay on the local CPU and progress is made.

Even native CAS architectures can fail to provide forward progress for their
primitive (See Sparc64 for an example).

Such implementations are strongly encouraged to add exponential backoff loops
to a failed CAS in order to ensure some progress. Affected architectures are
also strongly encouraged to inspect/audit the atomic fallbacks, refcount_t and
their locking primitives.