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@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: dave@stgolabs.net
Cc: linux-doc@vger.kernel.org
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1461691328-5429-2-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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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