[tip:locking/core] locking/Documentation: State purpose of memory-barriers.txt

[tip-bot for David Howells]

Commit-ID:  8d4840e84871847ee1bae56a776907d08a9265f7
Gitweb:     http://git.kernel.org/tip/8d4840e84871847ee1bae56a776907d08a9265f7
Author:     David Howells <dhowells@xxxxxxxxxx>
AuthorDate: Tue, 26 Apr 2016 10:22:06 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 28 Apr 2016 10:57:51 +0200

locking/Documentation: State purpose of memory-barriers.txt

There has been some confusion about the purpose of memory-barriers.txt,
so this commit adds a statement of purpose.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: corbet@xxxxxxx
Cc: dave@xxxxxxxxxxxx
Cc: linux-doc@xxxxxxxxxxxxxxx
Cc: will.deacon@xxxxxxx
Link: http://lkml.kernel.org/r/1461691328-5429-2-git-send-email-paulmck@xxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 Documentation/memory-barriers.txt | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index fb2dd35..8b11e54 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -19,6 +19,22 @@ in case of any doubt (and there are many) please ask.
 To repeat, this document is not a specification of what Linux expects from
 hardware.
 
+The purpose of this document is twofold:
+
+ (1) to specify the minimum functionality that one can rely on for any
+     particular barrier, and
+
+ (2) to provide a guide as to how to use the barriers that are available.
+
+Note that an architecture can provide more than the minimum requirement
+for any particular barrier, but if the architecure provides less than
+that, that architecture is incorrect.
+
+Note also that it is possible that a barrier may be a no-op for an
+architecture because the way that arch works renders an explicit barrier
+unnecessary in that case.
+
+
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 CONTENTS
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