tests-aio-multithread: use atomic_read properly

nodes[id].next is written by other threads.  If atomic_read is not used
(matching atomic_set in mcs_mutex_lock!) the compiler can optimize the
whole "if" away!
Reported-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Tested-by: NGreg Kurz <groug@kaod.org>
Message-id: 20170227111726.9237-1-pbonzini@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

tests/test-aio-multithread.c

@@ -309,7 +309,7 @@ static void mcs_mutex_lock(void)
 static void mcs_mutex_unlock(void)
 {
     int next;
-    if (nodes[id].next == -1) {
+    if (atomic_read(&nodes[id].next) == -1) {
         if (atomic_read(&mutex_head) == id &&
             atomic_cmpxchg(&mutex_head, id, -1) == id) {
             /* Last item in the list, exit.  */
@@ -323,7 +323,7 @@ static void mcs_mutex_unlock(void)
     }
 
     /* Wake up the next in line.  */
-    next = nodes[id].next;
+    next = atomic_read(&nodes[id].next);
     nodes[next].locked = 0;
     qemu_futex_wake(&nodes[next].locked, 1);
 }