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

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