Documentation/locking/atomic: Add documents for new atomic_t APIs

Since we've vastly expanded the atomic_t interface in recent years the
existing documentation is woefully out of date and people seem to get
confused a bit.

Start a new document to hopefully better explain the current state of
affairs.

The old atomic_ops.txt also covers bitmaps and a few more details so
this is not a full replacement and we'll therefore keep that document
around until such a time that we've managed to write more text to cover
its entire.

Also please, ReST people, go away.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 200 insertions, 0 deletions

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
new file mode 100644
index 000000000000..eee127115277
--- /dev/null
+++ b/Documentation/atomic_t.txt
@@ -0,0 +1,200 @@
+
+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:
+
+