[PATCH v4 4/4] Documentation/litmus-tests/atomic: Add a test for smp_mb__after_atomic()

Boqun Feng
2020-03-26T02:40:58Z

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.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
---
Documentation/atomic_t.txt | 10 +++---
...ter_atomic-is-stronger-than-acquire.litmus | 32 +++++++++++++++++++
Documentation/litmus-tests/atomic/README | 5 +++
3 files changed, 42 insertions(+), 5 deletions(-)
create mode 100644 Documentation/litmus-tests/atomic/Atomic-RMW+mb__after_atomic-is-stronger-than-acquire.litmus

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index 67d1d99f8589..0f1fdedf36bb 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -233,19 +233,19 @@ as well. Similarly, something like:
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();
@@ -253,14 +253,14 @@ strictly stronger than ACQUIRE. As illustrated:
}

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++;
diff --git a/Documentation/litmus-tests/atomic/Atomic-RMW+mb__after_atomic-is-stronger-than-acquire.litmus b/Documentation/litmus-tests/atomic/Atomic-RMW+mb__after_atomic-is-stronger-than-acquire.litmus
new file mode 100644
index 000000000000..9a8e31a44b28
--- /dev/null
+++ b/Documentation/litmus-tests/atomic/Atomic-RMW+mb__after_atomic-is-stronger-than-acquire.litmus
@@ -0,0 +1,32 @@
+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)
diff --git a/Documentation/litmus-tests/atomic/README b/Documentation/litmus-tests/atomic/README
index a1b72410b539..714cf93816ea 100644
--- a/Documentation/litmus-tests/atomic/README
+++ b/Documentation/litmus-tests/atomic/README
@@ -7,5 +7,10 @@ tools/memory-model/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.
--
2.25.1