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    SubjectRe: Soft-Lockup/Race in networking in 2.6.31-rc1+195 ( possibly?caused by netem)
    On Thu, Jul 09, 2009 at 12:31:53PM +0200, Thomas Gleixner wrote:
    > On Thu, 9 Jul 2009, Jarek Poplawski wrote:
    > > On Thu, Jul 09, 2009 at 12:23:17AM +0200, Andres Freund wrote:
    > > ...
    > > > Unfortunately this just yields the same backtraces during softlockup and not
    > > > earlier.
    > > > I did not test without lockdep yet, but that should not have stopped the BUG
    > > > from appearing, right?
    > >
    > > Since it looks like hrtimers now, these changes in timers shouldn't
    > > matter. Let's wait for new ideas.
    >
    > Some background:
    ...
    > There is another oddity in cbq_undelay() which is the hrtimer callback
    > function:
    >
    > if (delay) {
    > ktime_t time;
    >
    > time = ktime_set(0, 0);
    > time = ktime_add_ns(time, PSCHED_TICKS2NS(now + delay));
    > hrtimer_start(&q->delay_timer, time, HRTIMER_MODE_ABS);
    >
    > The canocial way to restart a hrtimer from the callback function is to
    > set the expiry value and return HRTIMER_RESTART.

    OK, that's for later because we didn't use cbq here.

    >
    > }
    >
    > sch->flags &= ~TCQ_F_THROTTLED;
    > __netif_schedule(qdisc_root(sch));
    > return HRTIMER_NORESTART;
    >
    > Again, this should not cause the timer to be enqueued on another CPU
    > as we do not enqueue on a different CPU when the callback is running,
    > but see above ...
    >
    > I have the feeling that the code relies on some implicit cpu
    > boundness, which is not longer guaranteed with the timer migration
    > changes, but that's a question for the network experts.

    As a matter of fact, I've just looked at this __netif_schedule(),
    which really is cpu bound, so you might be 100% right.

    Thanks for your help,
    Jarek P.


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