monitor: let suspend_cnt be thread safe

Monitor code now can be run in more than one thread.  Let it be thread
safe when accessing suspend_cnt counter.
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-13-peterx@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

monitor.c

@@ -194,7 +194,7 @@ struct Monitor {
     CharBackend chr;
     int reset_seen;
     int flags;
-    int suspend_cnt;
+    int suspend_cnt;            /* Needs to be accessed atomically */
     bool skip_flush;
     bool use_io_thr;
 
@@ -3819,7 +3819,7 @@ static int monitor_can_read(void *opaque)
 {
     Monitor *mon = opaque;
 
-    return (mon->suspend_cnt == 0) ? 1 : 0;
+    return !atomic_mb_read(&mon->suspend_cnt);
 }
 
 /*
@@ -3951,7 +3951,7 @@ int monitor_suspend(Monitor *mon)
 {
     if (!mon->rs)
         return -ENOTTY;
-    mon->suspend_cnt++;
+    atomic_inc(&mon->suspend_cnt);
     return 0;
 }
 
@@ -3959,8 +3959,9 @@ void monitor_resume(Monitor *mon)
 {
     if (!mon->rs)
         return;
-    if (--mon->suspend_cnt == 0)
+    if (atomic_dec_fetch(&mon->suspend_cnt) == 0) {
         readline_show_prompt(mon->rs);
+    }
 }
 
 static QObject *get_qmp_greeting(Monitor *mon)
@@ -4025,19 +4026,19 @@ static void monitor_event(void *opaque, int event)
             monitor_resume(mon);
             monitor_flush(mon);
         } else {
-            mon->suspend_cnt = 0;
+            atomic_mb_set(&mon->suspend_cnt, 0);
         }
         break;
 
     case CHR_EVENT_MUX_OUT:
         if (mon->reset_seen) {
-            if (mon->suspend_cnt == 0) {
+            if (atomic_mb_read(&mon->suspend_cnt) == 0) {
                 monitor_printf(mon, "\n");
             }
             monitor_flush(mon);
             monitor_suspend(mon);
         } else {
-            mon->suspend_cnt++;
+            atomic_inc(&mon->suspend_cnt);
         }
         qemu_mutex_lock(&mon->out_lock);
         mon->mux_out = 1;