Documentation/litmus-tests/atomic: Add a test for smp_mb__after_atomic()

We already use a litmus test in atomic_t.txt to describe atomic RMW +
smp_mb__after_atomic() is stronger than acquire (both the read and the
write parts are ordered). So make it a litmus test in atomic-tests
directory, so that people can access the litmus easily.

Additionally, change the processor numbers "P1, P2" to "P0, P1" in
atomic_t.txt for the consistency with the processor numbers in the
litmus test, which herd can handle.

Acked-by: Alan Stern <[e-mail]>
Acked-by: Andrea Parri <[e-mail]>
Signed-off-by: Boqun Feng <[e-mail]>
Reviewed-by: Joel Fernandes (Google) <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/atomic_t.txt

···
 is an ACQUIRE pattern (though very much not typical), but again the barrier is
 strictly stronger than ACQUIRE. As illustrated:
 
-  C strong-acquire
+  C Atomic-RMW+mb__after_atomic-is-stronger-than-acquire
 
   {
   }
 
-  P1(int *x, atomic_t *y)
+  P0(int *x, atomic_t *y)
   {
     r0 = READ_ONCE(*x);
     smp_rmb();
     r1 = atomic_read(y);
   }
 
-  P2(int *x, atomic_t *y)
+  P1(int *x, atomic_t *y)
   {
     atomic_inc(y);
     smp_mb__after_atomic();
···
   }
 
   exists
-  (r0=1 /\ r1=0)
+  (0:r0=1 /\ 0:r1=0)
 
 This should not happen; but a hypothetical atomic_inc_acquire() --
 (void)atomic_fetch_inc_acquire() for instance -- would allow the outcome,
 because it would not order the W part of the RMW against the following
 WRITE_ONCE.  Thus:
 
-  P1			P2
+  P0			P1
 
 			t = LL.acq *y (0)
 			t++;

Documentation/litmus-tests/atomic/Atomic-RMW+mb__after_atomic-is-stronger-than-acquire.litmus

···
+C Atomic-RMW+mb__after_atomic-is-stronger-than-acquire
+
+(*
+ * Result: Never
+ *
+ * Test that an atomic RMW followed by a smp_mb__after_atomic() is
+ * stronger than a normal acquire: both the read and write parts of
+ * the RMW are ordered before the subsequential memory accesses.
+ *)
+
+{
+}
+
+P0(int *x, atomic_t *y)
+{
+	int r0;
+	int r1;
+
+	r0 = READ_ONCE(*x);
+	smp_rmb();
+	r1 = atomic_read(y);
+}
+
+P1(int *x, atomic_t *y)
+{
+	atomic_inc(y);
+	smp_mb__after_atomic();
+	WRITE_ONCE(*x, 1);
+}
+
+exists
+(0:r0=1 /\ 0:r1=0)

Documentation/litmus-tests/atomic/README

···
 LITMUS TESTS
 ============
 
+Atomic-RMW+mb__after_atomic-is-stronger-than-acquire
+	Test that an atomic RMW followed by a smp_mb__after_atomic() is
+	stronger than a normal acquire: both the read and write parts of
+	the RMW are ordered before the subsequential memory accesses.
+
 Atomic-RMW-ops-are-atomic-WRT-atomic_set.litmus
 	Test that atomic_set() cannot break the atomicity of atomic RMWs.