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existing documentation is woefully out of date and people seem to get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>

Documentation/memory-barriers.txt

@@ -498,11 +498,11 @@ And a couple of implicit varieties:
      This means that ACQUIRE acts as a minimal "acquire" operation and
      RELEASE acts as a minimal "release" operation.
 
-A subset of the atomic operations described in core-api/atomic_ops.rst have
-ACQUIRE and RELEASE variants in addition to fully-ordered and relaxed (no
-barrier semantics) definitions.  For compound atomics performing both a load
-and a store, ACQUIRE semantics apply only to the load and RELEASE semantics
-apply only to the store portion of the operation.
+A subset of the atomic operations described in atomic_t.txt have ACQUIRE and
+RELEASE variants in addition to fully-ordered and relaxed (no barrier
+semantics) definitions.  For compound atomics performing both a load and a
+store, ACQUIRE semantics apply only to the load and RELEASE semantics apply
+only to the store portion of the operation.
 
 Memory barriers are only required where there's a possibility of interaction
 between two CPUs or between a CPU and a device.  If it can be guaranteed that
@@ -1876,8 +1876,7 @@ There are some more advanced barrier functions:
      This makes sure that the death mark on the object is perceived to be set
      *before* the reference counter is decremented.
 
-     See Documentation/core-api/atomic_ops.rst for more information.  See the
-     "Atomic operations" subsection for information on where to use these.
+     See Documentation/atomic_{t,bitops}.txt for more information.
 
 
  (*) lockless_dereference();
@@ -2503,88 +2502,7 @@ operations are noted specially as some of them imply full memory barriers and
 some don't, but they're very heavily relied on as a group throughout the
 kernel.
 
-Any atomic operation that modifies some state in memory and returns information
-about the state (old or new) implies an SMP-conditional general memory barrier
-(smp_mb()) on each side of the actual operation (with the exception of
-explicit lock operations, described later).  These include:
-
-	xchg();
-	atomic_xchg();			atomic_long_xchg();
-	atomic_inc_return();		atomic_long_inc_return();
-	atomic_dec_return();		atomic_long_dec_return();
-	atomic_add_return();		atomic_long_add_return();
-	atomic_sub_return();		atomic_long_sub_return();
-	atomic_inc_and_test();		atomic_long_inc_and_test();
-	atomic_dec_and_test();		atomic_long_dec_and_test();
-	atomic_sub_and_test();		atomic_long_sub_and_test();
-	atomic_add_negative();		atomic_long_add_negative();
-	test_and_set_bit();
-	test_and_clear_bit();
-	test_and_change_bit();
-
-	/* when succeeds */
-	cmpxchg();
-	atomic_cmpxchg();		atomic_long_cmpxchg();
-	atomic_add_unless();		atomic_long_add_unless();
-
-These are used for such things as implementing ACQUIRE-class and RELEASE-class
-operations and adjusting reference counters towards object destruction, and as
-such the implicit memory barrier effects are necessary.
-
-
-The following operations are potential problems as they do _not_ imply memory
-barriers, but might be used for implementing such things as RELEASE-class
-operations:
-
-	atomic_set();
-	set_bit();
-	clear_bit();
-	change_bit();
-
-With these the appropriate explicit memory barrier should be used if necessary
-(smp_mb__before_atomic() for instance).
-
-
-The following also do _not_ imply memory barriers, and so may require explicit
-memory barriers under some circumstances (smp_mb__before_atomic() for
-instance):
-
-	atomic_add();
-	atomic_sub();
-	atomic_inc();
-	atomic_dec();
-
-If they're used for statistics generation, then they probably don't need memory
-barriers, unless there's a coupling between statistical data.
-
-If they're used for reference counting on an object to control its lifetime,
-they probably don't need memory barriers because either the reference count
-will be adjusted inside a locked section, or the caller will already hold
-sufficient references to make the lock, and thus a memory barrier unnecessary.
-
-If they're used for constructing a lock of some description, then they probably
-do need memory barriers as a lock primitive generally has to do things in a
-specific order.
-
-Basically, each usage case has to be carefully considered as to whether memory
-barriers are needed or not.
-
-The following operations are special locking primitives:
-
-	test_and_set_bit_lock();
-	clear_bit_unlock();
-	__clear_bit_unlock();
-
-These implement ACQUIRE-class and RELEASE-class operations.  These should be
-used in preference to other operations when implementing locking primitives,
-because their implementations can be optimised on many architectures.
-
-[!] Note that special memory barrier primitives are available for these
-situations because on some CPUs the atomic instructions used imply full memory
-barriers, and so barrier instructions are superfluous in conjunction with them,
-and in such cases the special barrier primitives will be no-ops.
-
-See Documentation/core-api/atomic_ops.rst for more information.
+See Documentation/atomic_t.txt for more information.
 
 
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