tools/memory-model: Add a simple entry point document

Current LKMM documentation assumes that the reader already understands
concurrency in the Linux kernel, which won't necessarily always be the
case.  This commit supplies a simple.txt file that provides a starting
point for someone who is new to concurrency in the Linux kernel.
That said, this file might also useful as a reminder to experienced
developers of simpler approaches to dealing with concurrency.

Link: Link: https://lwn.net/Articles/827180/
[ paulmck: Apply feedback from Joel Fernandes. ]
Co-developed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Co-developed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/memory-model/Documentation/litmus-tests.txt

@@ -726,8 +726,12 @@ P0()'s line 10 initializes "x" to the value 1 then line 11 links to "x"
 from "y", replacing "z".
 
 P1()'s line 20 loads a pointer from "y", and line 21 dereferences that
-pointer.  The RCU read-side critical section spanning lines 19-22 is
-just for show in this example.
+pointer.  The RCU read-side critical section spanning lines 19-22 is just
+for show in this example.  Note that the address used for line 21's load
+depends on (in this case, "is exactly the same as") the value loaded by
+line 20.  This is an example of what is called an "address dependency".
+This particular address dependency extends from the load on line 20 to the
+load on line 21.  Address dependencies provide a weak form of ordering.
 
 Running this test results in the following:
 

tools/memory-model/README

@@ -177,6 +177,11 @@ Documentation/recipes.txt
 Documentation/references.txt
 	Provides background reading.
 
+Documentation/simple.txt
+	Starting point for someone new to Linux-kernel concurrency.
+	And also for those needing a reminder of the simpler approaches
+	to concurrency!
+
 linux-kernel.bell
 	Categorizes the relevant instructions, including memory
 	references, memory barriers, atomic read-modify-write operations,