tools/memory-model: Add a S lock-based external-view litmus test

This commit adds a litmus test in which P0() and P1() form a lock-based S
litmus test, with the addition of P2(), which observes P0()'s and P1()'s
accesses with a full memory barrier but without the lock. This litmus
test asks whether writes carried out by two different processes under the
same lock will be seen in order by a third process not holding that lock.
The answer to this question is "yes" for all architectures supporting
the Linux kernel, but is "no" according to the current version of LKMM.

A patch to LKMM is under development.

Signed-off-by: Alan Stern <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Acked-by: Peter Zijlstra <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus

···
+C ISA2+pooncelock+pooncelock+pombonce.litmus
+
+(*
+ * Result: Sometimes
+ *
+ * This test shows that the ordering provided by a lock-protected S
+ * litmus test (P0() and P1()) are not visible to external process P2().
+ * This is likely to change soon.
+ *)
+
+{}
+
+P0(int *x, int *y, spinlock_t *mylock)
+{
+	spin_lock(mylock);
+	WRITE_ONCE(*x, 1);
+	WRITE_ONCE(*y, 1);
+	spin_unlock(mylock);
+}
+
+P1(int *y, int *z, spinlock_t *mylock)
+{
+	int r0;
+
+	spin_lock(mylock);
+	r0 = READ_ONCE(*y);
+	WRITE_ONCE(*z, 1);
+	spin_unlock(mylock);
+}
+
+P2(int *x, int *z)
+{
+	int r1;
+	int r2;
+
+	r2 = READ_ONCE(*z);
+	smp_mb();
+	r1 = READ_ONCE(*x);
+}
+
+exists (1:r0=1 /\ 2:r2=1 /\ 2:r1=0)