qemu-coroutine: remove qatomic_mb_read()

Replace with an explicit barrier and a comment.

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

util/qemu-coroutine.c

@@ -127,9 +127,13 @@ void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co)
         Coroutine *to = QSIMPLEQ_FIRST(&pending);
         CoroutineAction ret;
 
-        /* Cannot rely on the read barrier for to in aio_co_wake(), as there are
-         * callers outside of aio_co_wake() */
-        const char *scheduled = qatomic_mb_read(&to->scheduled);
+        /*
+         * Read to before to->scheduled; pairs with qatomic_cmpxchg in
+         * qemu_co_sleep(), aio_co_schedule() etc.
+         */
+        smp_read_barrier_depends();
+
+        const char *scheduled = qatomic_read(&to->scheduled);
 
         QSIMPLEQ_REMOVE_HEAD(&pending, co_queue_next);