Thanks for your comments!

Patrick McHardy wrote:

Ariane Keller wrote:

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That sounds like it would also be possible using rtnetlink. You could
send out a notification whenever you switch the active buffer and have
userspace listen to these and replace the inactive one.

I guess using rtnetlink is possible. However I'm not sure about how to 
implement it:
The first thought was to use RTM_NEWQDISC to send the data to the 
netem_change() function (similar to tc_qdisc_modify() ).

That sounds reasonable.

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But with this we would need the tcm_handle, tcm_parent arguments etc. 
which are not known in q_netem.c
Therefore we would have to change the parse_qopt() function prototype 
in order to pass the whole "req" and not only the nlmsghdr.

I assume you mean netem_init, parse_qopt is userspace. But I don't
see how that is related, emptying the buffer happens during packet
processing, right?

Actually I meant parse_qopt from user space.
If we would change that function prototype we would have the whole 
message header available in netem_parse_opt() and could pass this to the 
process which is responsible for sending the data to the kernel. This 
process would use this header every time it has to send new values to 
the netem_change() function in the kernel module.

I thought about this because I was not aware of the qdisc_notify function.
Anyway I've got some troubles with calling qdisc_notify.
1. I have to do a EXPORT_SYMBOL(qdisc_notify) (currently it is declared 
static in sch_api.c)
2. I'd like to call it from netem_enqueue(), which leads to a "sleeping 
function called from invalid context", since we are still in interrupt 
context. Therefore I think I have to put it in a workqueue.

I hope, this is ok.

I guess I would simply change the qdisc_notify function to not
require a struct nlmsghdr * (simply pass nlmsg_seq directly) and
use that to send notifications. The netem dump function would
add the buffer state. BTW, the parent class id is available in
sch->parent, the handle in sch->handle, but qdisc_notify should
take care of everything you need.