test-aio-multithread: simplify test_multi_co_schedule

Instead of using qatomic_mb_{read,set} mindlessly, just use a per-coroutine
flag that requires no synchronization.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/unit/test-aio-multithread.c

@@ -107,8 +107,7 @@ static void test_lifecycle(void)
 /* aio_co_schedule test.  */
 
 static Coroutine *to_schedule[NUM_CONTEXTS];
-
-static bool now_stopping;
+static bool stop[NUM_CONTEXTS];
 
 static int count_retry;
 static int count_here;
@@ -136,20 +135,27 @@ static bool schedule_next(int n)
 
 static void finish_cb(void *opaque)
 {
+    stop[id] = true;
     schedule_next(id);
 }
 
 static coroutine_fn void test_multi_co_schedule_entry(void *opaque)
 {
     g_assert(to_schedule[id] == NULL);
 
-    while (!qatomic_mb_read(&now_stopping)) {
+    /*
+     * The next iteration will set to_schedule[id] again, but once finish_cb
+     * is scheduled there is no guarantee that it will actually be woken up,
+     * so at that point it must not go to sleep.
+     */
+    while (!stop[id]) {
         int n;
 
         n = g_test_rand_int_range(0, NUM_CONTEXTS);
         schedule_next(n);
 
         qatomic_mb_set(&to_schedule[id], qemu_coroutine_self());
+        /* finish_cb can run here.  */
         qemu_coroutine_yield();
         g_assert(to_schedule[id] == NULL);
     }
@@ -161,7 +167,6 @@ static void test_multi_co_schedule(int seconds)
     int i;
 
     count_here = count_other = count_retry = 0;
-    now_stopping = false;
 
     create_aio_contexts();
     for (i = 0; i < NUM_CONTEXTS; i++) {
@@ -171,10 +176,10 @@ static void test_multi_co_schedule(int seconds)
 
     g_usleep(seconds * 1000000);
 
-    qatomic_mb_set(&now_stopping, true);
+    /* Guarantee that each AioContext is woken up from its last wait.  */
     for (i = 0; i < NUM_CONTEXTS; i++) {
         ctx_run(i, finish_cb, NULL);
-        to_schedule[i] = NULL;
+        g_assert(to_schedule[i] == NULL);
     }
 
     join_aio_contexts();
@@ -199,6 +204,7 @@ static uint32_t atomic_counter;
 static uint32_t running;
 static uint32_t counter;
 static CoMutex comutex;
+static bool now_stopping;
 
 static void coroutine_fn test_multi_co_mutex_entry(void *opaque)
 {