util: Fix order of operands and carry of one second in timespec_diff_us()

If the nanoseconds of the minuend timestamp are less than the
nanoseconds of the subtrahend timestamp, we need to carry one second
in the subtraction.

I subtracted this second from the minuend, but didn't actually carry
it in the subtraction of nanoseconds, and logged timestamps would jump
back whenever we switched to the first branch of timespec_diff_us()
from the second one.

Most likely, the reason why I didn't carry the second is that I
instinctively thought that swapping the operands would have the same
effect. But it doesn't, in general: that only happens with arithmetic
in modulo powers of 2. Undo the swap as well.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 1 insertions, 1 deletions

diff --git a/util.c b/util.c
index 6e64279..eede4e5 100644
--- a/util.c
+++ b/util.c
@@ -249,7 +249,7 @@ void sock_probe_mem(struct ctx *c)
 int64_t timespec_diff_us(const struct timespec *a, const struct timespec *b)
 {
 	if (a->tv_nsec < b->tv_nsec) {
-		return (b->tv_nsec - a->tv_nsec) / 1000 +
+		return (a->tv_nsec + 1000000000 - b->tv_nsec) / 1000 +
 		       (a->tv_sec - b->tv_sec - 1) * 1000000;
 	}