Fix: handle negative clock offset for lttng-ust metadata

[lttng-tools.git] / src / bin / lttng-sessiond / ust-metadata.c

diff --git a/src/bin/lttng-sessiond/ust-metadata.c b/src/bin/lttng-sessiond/ust-metadata.c
index 984d5f813ccf9277e6e221b7299f8f80a7d50954..276dc7a4414043f9c177abe75bf11a85b50c2c5f 100644 (file)
--- a/src/bin/lttng-sessiond/ust-metadata.c
+++ b/src/bin/lttng-sessiond/ust-metadata.c
@@ -604,11 +604,17 @@ int _lttng_event_header_declare(struct ust_registry_session *session)
        );
 }
 
+/*
+ * The offset between monotonic and realtime clock can be negative if
+ * the system sets the REALTIME clock to 0 after boot.
+ * Currently handle this by flooring the offset at 0.
+ */
 static
 int measure_single_clock_offset(struct offset_sample *sample)
 {
-       uint64_t offset, monotonic[2], measure_delta, realtime;
+       uint64_t monotonic_avg, monotonic[2], measure_delta, realtime;
        uint64_t tcf = trace_clock_freq();
+       int64_t offset;
        struct timespec rts = { 0, 0 };
        int ret;
 
@@ -626,15 +632,15 @@ int measure_single_clock_offset(struct offset_sample *sample)
                 */
                return 0;
        }
-       offset = (monotonic[0] + monotonic[1]) >> 1;
+       monotonic_avg = (monotonic[0] + monotonic[1]) >> 1;
        realtime = (uint64_t) rts.tv_sec * tcf;
        if (tcf == NSEC_PER_SEC) {
                realtime += rts.tv_nsec;
        } else {
                realtime += (uint64_t) rts.tv_nsec * tcf / NSEC_PER_SEC;
        }
-       offset = realtime - offset;
-       sample->offset = offset;
+       offset = (int64_t) realtime - monotonic_avg;
+       sample->offset = max(offset, 0);
        sample->measure_delta = measure_delta;
        return 0;
 }