tests-aio-multithread: fix /aio/multi/schedule race condition

test_multi_co_schedule_entry() set to_schedule[id] in the final loop
iteration before terminating the coroutine.  There is a race condition
where the main thread attempts to enter the terminating or terminated
coroutine when signalling coroutines to stop:

  atomic_mb_set(&now_stopping, true);
  for (i = 0; i < NUM_CONTEXTS; i++) {
      ctx_run(i, finish_cb, NULL);  <--- enters dead coroutine!
      to_schedule[i] = NULL;
  }

Make sure only to set to_schedule[id] if this coroutine really needs to
be scheduled!

Reported-by: "R.Nageswara Sastry" <nasastry@in.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20171106190233.1175-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 2 insertions, 3 deletions

diff --git a/tests/test-aio-multithread.c b/tests/test-aio-multithread.c
index 549d784915..d396185972 100644
--- a/tests/test-aio-multithread.c
+++ b/tests/test-aio-multithread.c
@@ -144,17 +144,16 @@ static void finish_cb(void *opaque)
 static coroutine_fn void test_multi_co_schedule_entry(void *opaque)
 {
     g_assert(to_schedule[id] == NULL);
-    atomic_mb_set(&to_schedule[id], qemu_coroutine_self());
 
     while (!atomic_mb_read(&now_stopping)) {
         int n;
 
         n = g_test_rand_int_range(0, NUM_CONTEXTS);
         schedule_next(n);
-        qemu_coroutine_yield();
 
-        g_assert(to_schedule[id] == NULL);
         atomic_mb_set(&to_schedule[id], qemu_coroutine_self());
+        qemu_coroutine_yield();
+        g_assert(to_schedule[id] == NULL);
     }
 }