[PATCH v2 4/6] test-coroutine: Add rwlock downgrade test

[David Edmondson <david.edmondson@oracle.com>]

Test that downgrading an rwlock does not result in a failure to
schedule coroutines queued on the rwlock.

The diagram associated with test_co_rwlock_downgrade() describes the
intended behaviour, but what is observed currently corresponds to:

| c1     | c2         | c3         | c4       |
|--------+------------+------------+----------|
| rdlock |            |            |          |
| yield  |            |            |          |
|        | wrlock     |            |          |
|        | <queued>   |            |          |
|        |            | rdlock     |          |
|        |            | <queued>   |          |
|        |            |            | wrlock   |
|        |            |            | <queued> |
| unlock |            |            |          |
| yield  |            |            |          |
|        | <dequeued> |            |          |
|        | downgrade  |            |          |
|        | ...        |            |          |
|        | unlock     |            |          |
|        |            | <dequeued> |          |
|        |            | <queued>   |          |

In the test, this results in a failure...

ERROR:../tests/test-coroutine.c:369:test_co_rwlock_downgrade: assertion failed: (c3_done)
Bail out! ERROR:../tests/test-coroutine.c:369:test_co_rwlock_downgrade: assertion failed: (c3_done)

...as a result of the c3 coroutine failing to run to completion.

Signed-off-by: David Edmondson <david.edmondson@oracle.com>
---
 tests/test-coroutine.c | 112 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)

diff --git a/tests/test-coroutine.c b/tests/test-coroutine.c
index e946d93a65..62b0092721 100644
--- a/tests/test-coroutine.c
+++ b/tests/test-coroutine.c
@@ -264,6 +264,117 @@ static void test_co_mutex_lockable(void)
     g_assert(QEMU_MAKE_LOCKABLE(null_pointer) == NULL);
 }
 
+static bool c1_done;
+static bool c2_done;
+static bool c3_done;
+static bool c4_done;
+
+static void coroutine_fn rwlock_c1(void *opaque)
+{
+    CoRwlock *l = opaque;
+
+    qemu_co_rwlock_rdlock(l);
+    qemu_coroutine_yield();
+
+    qemu_co_rwlock_unlock(l);
+    qemu_coroutine_yield();
+
+    c1_done = true;
+}
+
+static void coroutine_fn rwlock_c2(void *opaque)
+{
+    CoRwlock *l = opaque;
+
+    qemu_co_rwlock_wrlock(l);
+
+    qemu_co_rwlock_downgrade(l);
+    qemu_co_rwlock_unlock(l);
+    c2_done = true;
+}
+
+static void coroutine_fn rwlock_c3(void *opaque)
+{
+    CoRwlock *l = opaque;
+
+    qemu_co_rwlock_rdlock(l);
+
+    qemu_co_rwlock_unlock(l);
+    c3_done = true;
+}
+
+static void coroutine_fn rwlock_c4(void *opaque)
+{
+    CoRwlock *l = opaque;
+
+    qemu_co_rwlock_wrlock(l);
+
+    qemu_co_rwlock_unlock(l);
+    c4_done = true;
+}
+
+/*
+ * Check that downgrading a reader-writer lock does not cause a hang.
+ *
+ * Four coroutines are used to produce a situation where there are
+ * both reader and writer hopefuls waiting to acquire an rwlock that
+ * is held by a reader.
+ *
+ * The correct sequence of operations we aim to provoke can be
+ * represented as:
+ *
+ * | c1     | c2         | c3         | c4         |
+ * |--------+------------+------------+------------|
+ * | rdlock |            |            |            |
+ * | yield  |            |            |            |
+ * |        | wrlock     |            |            |
+ * |        | <queued>   |            |            |
+ * |        |            | rdlock     |            |
+ * |        |            | <queued>   |            |
+ * |        |            |            | wrlock     |
+ * |        |            |            | <queued>   |
+ * | unlock |            |            |            |
+ * | yield  |            |            |            |
+ * |        | <dequeued> |            |            |
+ * |        | downgrade  |            |            |
+ * |        | ...        |            |            |
+ * |        | unlock     |            |            |
+ * |        |            |            | <dequeued> |
+ * |        |            |            | unlock     |
+ * |        |            | <dequeued> |            |
+ * |        |            | unlock     |            |
+ *
+ * Significantly, when c2 unlocks the downgraded lock, it should be c4
+ * that is run rather than c3.
+ */
+static void test_co_rwlock_downgrade(void)
+{
+    CoRwlock l;
+    Coroutine *c1, *c2, *c3, *c4;
+
+    qemu_co_rwlock_init(&l);
+
+    c1 = qemu_coroutine_create(rwlock_c1, &l);
+    c2 = qemu_coroutine_create(rwlock_c2, &l);
+    c3 = qemu_coroutine_create(rwlock_c3, &l);
+    c4 = qemu_coroutine_create(rwlock_c4, &l);
+
+    qemu_coroutine_enter(c1);
+    qemu_coroutine_enter(c2);
+    qemu_coroutine_enter(c3);
+    qemu_coroutine_enter(c4);
+
+    qemu_coroutine_enter(c1);
+
+    g_assert(c2_done);
+    g_assert(c3_done);
+    g_assert(c4_done);
+
+    qemu_coroutine_enter(c1);
+
+    g_assert(c1_done);
+}
+
 /*
  * Check that creation, enter, and return work
  */
@@ -501,6 +612,7 @@ int main(int argc, char **argv)
     g_test_add_func("/basic/order", test_order);
     g_test_add_func("/locking/co-mutex", test_co_mutex);
     g_test_add_func("/locking/co-mutex/lockable", test_co_mutex_lockable);
+    g_test_add_func("/locking/co-rwlock/downgrade", test_co_rwlock_downgrade);
     if (g_test_perf()) {
         g_test_add_func("/perf/lifecycle", perf_lifecycle);
         g_test_add_func("/perf/nesting", perf_nesting);
-- 
2.30.1