[PATCH v6 tip/core/locking] Memory-barrier documentation updates + smp_mb__after_unlock_lock()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

Hello!

This series applies some long-needed updates to memory-barriers.txt and
adds an smp_mb__after_unlock_lock():

1.	Add ACCESS_ONCE() calls where needed to ensure their inclusion
	in code copy-and-pasted from this file.

2.	Add long atomic examples alongside the existing atomics.

3.	Prohibit architectures supporting the Linux kernel from
	speculating stores.

4.	Document what ACCESS_ONCE() does along with a number of situations
	requiring its use.

5.	Added smp_mb__after_unlock_lock() for all architectures.  Because
	all architectures are presumably providing full-barrier semantics
	for UNLOCK+LOCK, these are all no-ops (but see #8 below).  Some
	will change if low-latency-handoff queued locks are accepted.

6.	Downgrade UNLOCK+LOCK to no longer imply a full barrier, at least
	in the absence of smp_mb__after_unlock_lock().  See the LKML thread
	that includes http://www.spinics.net/lists/linux-mm/msg65653.html
	for more information.

7.	Applied smp_mb__after_unlock_lock() as needed in RCU.

8.	Make smp_mb__after_unlock_lock() be a full memory barrier for powerpc.

Changes from v5:

o	Added #8 (full memory barrier for powerpc).

o	Made the definition of smp_mb__after_unlock_lock() precede its
	documentation as suggested by Josh Triplett.

o	Added verbiage describing acquire and release semantics of
	LOCK and UNLOCK, respectively.

o	Updates based on Oleg Nesterov review.

Changes from v4:

o	Added Josh Triplett's Reviewed-by for 1-4.

o	Applied feedback from Ingo Molnar and Jonathan Corbet.

o	Trimmed Cc lists as suggested by David Miller.

o	Added smp_mb__after_unlock_lock() changes.

Changes from v3:

o	Fix typos noted by Peter Zijlstra.

o	Added the documentation about ACCESS_ONCE(), which expands on
	http://thread.gmane.org/gmane.linux.kernel.mm/82891/focus=14696,
	ably summarized by Jon Corbet at http://lwn.net/Articles/508991/.

Changes from v2:

o	Update examples so that that load against which the subsequent
	store is to be ordered is part of the "if" condition.

o	Add an example showing how the compiler can remove "if"
	conditions and how to prevent it from doing so.

o	Add ACCESS_ONCE() to the compiler-barrier section.

o	Add a sentence noting that transitivity requires smp_mb().

Changes from v1:

o	Combined with Peter Zijlstra's speculative-store-prohibition patch.

o	Added more pitfalls to avoid when prohibiting speculative
	stores, along with how to avoid them.

o	Applied Josh Triplett's review comments.

							Thanx, Paul

------------------------------------------------------------------------