trace: Direct access of atomics is verboten, use the API

The GLib Reference Manual says:

    It is very important that all accesses to a particular integer or
    pointer be performed using only this API and that different sizes
    of operation are not mixed or used on overlapping memory
    regions. Never read or assign directly from or to a value --
    always use this API.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NHarsh Prateek Bora <harsh@linux.vnet.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

trace/simple.c

@@ -166,13 +166,13 @@ static gpointer writeout_thread(gpointer opaque)
     for (;;) {
         wait_for_trace_records_available();
 
-        if (dropped_events) {
+        if (g_atomic_int_get(&dropped_events)) {
             dropped.rec.event = DROPPED_EVENT_ID,
             dropped.rec.timestamp_ns = get_clock();
             dropped.rec.length = sizeof(TraceRecord) + sizeof(uint64_t),
             dropped.rec.reserved = 0;
             while (1) {
-                dropped_count = dropped_events;
+                dropped_count = g_atomic_int_get(&dropped_events);
                 if (g_atomic_int_compare_and_exchange(&dropped_events,
                                                       dropped_count, 0)) {
                     break;
@@ -214,7 +214,7 @@ int trace_record_start(TraceBufferRecord *rec, TraceEventID event, size_t datasi
     uint64_t timestamp_ns = get_clock();
 
     while (1) {
-        old_idx = trace_idx;
+        old_idx = g_atomic_int_get(&trace_idx);
         smp_rmb();
         new_idx = old_idx + rec_len;
 
@@ -275,7 +275,8 @@ void trace_record_finish(TraceBufferRecord *rec)
     record.event |= TRACE_RECORD_VALID;
     write_to_buffer(rec->tbuf_idx, &record, sizeof(TraceRecord));
 
-    if ((trace_idx - writeout_idx) > TRACE_BUF_FLUSH_THRESHOLD) {
+    if ((g_atomic_int_get(&trace_idx) - writeout_idx)
+        > TRACE_BUF_FLUSH_THRESHOLD) {
         flush_trace_file(false);
     }
 }