Documentation/litmus-tests: Add locking tests to README

This commit documents the litmus tests in the "locking" directory.

[ paulmck: Apply formatting feedback from Andrea Parri. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Daniel Lustig <dlustig@nvidia.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: <linux-arch@vger.kernel.org>
Cc: <linux-doc@vger.kernel.org>
Acked-by: Andrea Parri <parri.andrea@gmail.com>

Author: paulmckrcu

Documentation/litmus-tests/README

@@ -22,6 +22,35 @@ Atomic-RMW-ops-are-atomic-WRT-atomic_set.litmus
     NOTE: Require herd7 7.56 or later which supports "(void)expr".
 
 
+locking (/locking directory)
+----------------------------
+
+DCL-broken.litmus
+    Demonstrates that double-checked locking needs more than just
+    the obvious lock acquisitions and releases.
+
+DCL-fixed.litmus
+    Demonstrates corrected double-checked locking that uses
+    smp_store_release() and smp_load_acquire() in addition to the
+    obvious lock acquisitions and releases.
+
+RM-broken.litmus
+    Demonstrates problems with "roach motel" locking, where code is
+    freely moved into lock-based critical sections.  This example also
+    shows how to use the "filter" clause to discard executions that
+    would be excluded by other code not modeled in the litmus test.
+    Note also that this "roach motel" optimization is emulated by
+    physically moving P1()'s two reads from x under the lock.
+
+    What is a roach motel?  This is from an old advertisement for
+    a cockroach trap, much later featured in one of the "Men in
+    Black" movies.  "The roaches check in.  They don't check out."
+
+RM-fixed.litmus
+    The counterpart to RM-broken.litmus, showing P0()'s two loads from
+    x safely outside of the critical section.
+
+
 RCU (/rcu directory)
 --------------------