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    SubjectRe: net_timedelta() affected by settimeofday() (was: [patch 12/13] net: mac80211: Remove silly timespec dance)
    On Thu, 12 Jun 2014, Johannes Berg wrote:

    > On Thu, 2014-06-12 at 10:35 +0200, Johannes Berg wrote:
    > > +netdev, Stephen
    >
    > Well, stupid me. Fixing that netdev address.
    >
    > > On Thu, 2014-06-12 at 10:19 +0200, Thomas Gleixner wrote:
    > > > On Thu, 12 Jun 2014, Johannes Berg wrote:
    > > >
    > > > > On Wed, 2014-06-11 at 23:59 +0000, Thomas Gleixner wrote:
    > > > >
    > > > > > + msrmnt = ktime_to_ms(net_timedelta(skb_arv));
    > > > >
    > > > > This is probably more of a question about net_timedelta(), but is
    > > > > ktime_get_real() really appropriate for duration measurements? Isn't
    > > > > that non-monotonic?
    > > >
    > > > Well, it's monotonic, but might be affected by settimeofday().
    > >
    > > Right, but isn't that odd? Suddenly your delay measurement here might be
    > > minutes, hours, or years if you settimeofday() between timestamping and
    > > calculating the delta. That seems very strange to me, why would that be
    > > the right behaviour in any way?

    Indeed. clock monotonic is the appropriate one for measurements.

    > > Now, it seems that there are only two current users of net_timedelta()
    > > (in DCCP) so perhaps it's not too late to change some of this?
    > >
    > > Maybe in general the skb timestamp should be based on a different clock
    > > and only adjusted to real time when used in userspace?

    You have the same problem then, just at a different place:

    ts = ktime_get();

    settimeofday()
    offset_mono_to_real = new value;

    userts = mono_to_real(ts);

    But maybe that's not a real issue, as ktime_get_real() can race with
    settimeofday() or NTP as well.

    ts = ktime_get_real();
    settimeofday();
    userts = ts;

    So the user might see a weird timestamp for a packet, which cannot be
    correlated with the user space gettimeofday().

    Thanks,

    tglx





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