[tip:locking/core] tools/memory-model: Add a S lock-based external-view litmus test

[tip-bot for Alan Stern]

Commit-ID:  556bb7d252ae42d4653557325670e665087c38ad
Gitweb:     https://git.kernel.org/tip/556bb7d252ae42d4653557325670e665087c38ad
Author:     Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
AuthorDate: Tue, 20 Feb 2018 15:25:10 -0800
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Wed, 21 Feb 2018 09:58:15 +0100

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 <stern@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: akiyks@xxxxxxxxx
Cc: boqun.feng@xxxxxxxxx
Cc: dhowells@xxxxxxxxxx
Cc: j.alglave@xxxxxxxxx
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: luc.maranget@xxxxxxxx
Cc: nborisov@xxxxxxxx
Cc: npiggin@xxxxxxxxx
Cc: parri.andrea@xxxxxxxxx
Cc: will.deacon@xxxxxxx
Link: http://lkml.kernel.org/r/1519169112-20593-10-git-send-email-paulmck@xxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 ...ce.litmus => ISA2+pooncelock+pooncelock+pombonce.litmus} | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus b/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
similarity index 55%
copy from tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus
copy to tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
index 10a2aa0..7a39a0a 100644
--- a/tools/memory-model/litmus-tests/Z6.0+pooncelock+pooncelock+pombonce.litmus
+++ b/tools/memory-model/litmus-tests/ISA2+pooncelock+pooncelock+pombonce.litmus
@@ -1,11 +1,11 @@
-C Z6.0+pooncelock+pooncelock+pombonce
+C ISA2+pooncelock+pooncelock+pombonce.litmus
 
 (*
  * Result: Sometimes
  *
- * This example demonstrates that a pair of accesses made by different
- * processes each while holding a given lock will not necessarily be
- * seen as ordered by a third process not holding that lock.
+ * 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.
  *)
 
 {}
@@ -31,10 +31,11 @@ P1(int *y, int *z, spinlock_t *mylock)
 P2(int *x, int *z)
 {
 	int r1;
+	int r2;
 
-	WRITE_ONCE(*z, 2);
+	r2 = READ_ONCE(*z);
 	smp_mb();
 	r1 = READ_ONCE(*x);
 }
 
-exists (1:r0=1 /\ z=2 /\ 2:r1=0)
+exists (1:r0=1 /\ 2:r2=1 /\ 2:r1=0)