tools/memory-model: Document categories of ordering primitives

The Linux kernel has a number of categories of ordering primitives, which
are recorded in the LKMM implementation and hinted at by cheatsheet.txt.
But there is no overview of these categories, and such an overview
is needed in order to understand multithreaded LKMM litmus tests.
This commit therefore adds an ordering.txt as well as extracting a
control-dependencies.txt from memory-barriers.txt.  It also updates the
README file.

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[ paulmck:  Apply Alan Stern feedback. ]
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Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/memory-model/Documentation/README

@@ -11,6 +11,12 @@ the material provided by documents earlier in this list.
 
 o	You are new to Linux-kernel concurrency: simple.txt
 
+o	You have some background in Linux-kernel concurrency, and would
+	like an overview of the types of low-level concurrency primitives
+	that the Linux kernel provides:  ordering.txt
+
+	Here, "low level" means atomic operations to single variables.
+
 o	You are familiar with the Linux-kernel concurrency primitives
 	that you need, and just want to get started with LKMM litmus
 	tests:  litmus-tests.txt
@@ -19,6 +25,9 @@ o	You are familiar with Linux-kernel concurrency, and would
 	like a detailed intuitive understanding of LKMM, including
 	situations involving more than two threads:  recipes.txt
 
+o	You would like a detailed understanding of what your compiler can
+	and cannot do to control dependencies:  control-dependencies.txt
+
 o	You are familiar with Linux-kernel concurrency and the use of
 	LKMM, and would like a quick reference:  cheatsheet.txt
 
@@ -41,6 +50,10 @@ README
 cheatsheet.txt
 	Quick-reference guide to the Linux-kernel memory model.
 
+control-dependencies.txt
+	Guide to preventing compiler optimizations from destroying
+	your control dependencies.
+
 explanation.txt
 	Detailed description of the memory model.
 
@@ -48,6 +61,10 @@ litmus-tests.txt
 	The format, features, capabilities, and limitations of the litmus
 	tests that LKMM can evaluate.
 
+ordering.txt
+	Overview of the Linux kernel's low-level memory-ordering
+	primitives by category.
+
 recipes.txt
 	Common memory-ordering patterns.