From 2b644192779b07e8de2285ff5f68903c6c70597f Mon Sep 17 00:00:00 2001 From: Mathieu Desnoyers Date: Mon, 8 Feb 2016 16:38:46 -0500 Subject: [PATCH] Fix: handle negative clock offset In the unlikely situation where a system sets its hardware clock (CLOCK_REALTIME) to 0 (Epoch) after boot, the difference monotonic - realtime becomes negative. Fixup this situation by returning a 0 offset in this case. This ensures that trace viewer implementations (e.g. babeltrace) which currently cannot handle the negative offset (known bug) still work with the generated traces. Signed-off-by: Mathieu Desnoyers --- lttng-events.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/lttng-events.c b/lttng-events.c index d0ebb298..aac9c589 100644 --- a/lttng-events.c +++ b/lttng-events.c @@ -1960,11 +1960,14 @@ int _lttng_event_header_declare(struct lttng_session *session) * taken at start of trace. * Yes, this is only an approximation. Yes, we can (and will) do better * in future versions. + * Return 0 if offset is negative. It may happen if the system sets + * the REALTIME clock to 0 after boot. */ static uint64_t measure_clock_offset(void) { - uint64_t offset, monotonic[2], realtime; + uint64_t monotonic_avg, monotonic[2], realtime; + int64_t offset; struct timespec rts = { 0, 0 }; unsigned long flags; @@ -1975,10 +1978,12 @@ uint64_t measure_clock_offset(void) monotonic[1] = trace_clock_read64(); local_irq_restore(flags); - offset = (monotonic[0] + monotonic[1]) >> 1; + monotonic_avg = (monotonic[0] + monotonic[1]) >> 1; realtime = (uint64_t) rts.tv_sec * NSEC_PER_SEC; realtime += rts.tv_nsec; - offset = realtime - offset; + offset = (int64_t) realtime - monotonic_avg; + if (offset < 0) + return 0; return offset; } -- 2.34.1