tools/memory-model: Add an exception for limitations on _unless() family

According to Luc, atomic_add_unless() is directly provided by herd7,
therefore it can be used in litmus tests. So change the limitation
section in README to unlimit the use of atomic_add_unless().

Cc: Luc Maranget <[email protected]>
Acked-by: Andrea Parri <[email protected]>
Reviewed-by: Joel Fernandes (Google) <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: fbq

tools/memory-model/README

@@ -207,11 +207,15 @@ The Linux-kernel memory model (LKMM) has the following limitations:
 		case as a store release.
 
 	b.	The "unless" RMW operations are not currently modeled:
-		atomic_long_add_unless(), atomic_add_unless(),
-		atomic_inc_unless_negative(), and
-		atomic_dec_unless_positive().  These can be emulated
+		atomic_long_add_unless(), atomic_inc_unless_negative(),
+		and atomic_dec_unless_positive().  These can be emulated
 		in litmus tests, for example, by using atomic_cmpxchg().
 
+		One exception of this limitation is atomic_add_unless(),
+		which is provided directly by herd7 (so no corresponding
+		definition in linux-kernel.def).  atomic_add_unless() is
+		modeled by herd7 therefore it can be used in litmus tests.
+
 	c.	The call_rcu() function is not modeled.  It can be
 		emulated in litmus tests by adding another process that
 		invokes synchronize_rcu() and the body of the callback