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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