On Fri, Jun 26, 2020 at 3:46 PM Petr Machata [off-list ref] wrote:

The function tcf_qevent_handle() should be invoked when qdisc hits the
"interesting event" corresponding to a block. This function releases root
lock for the duration of executing the attached filters, to allow packets
generated through user actions (notably mirred) to be reinserted to the
same qdisc tree.

I read this again, another question here is: why is tcf_qevent_handle()
special here? We call tcf_classify() under root lock in many other places
too, for example htb_enqueue(), which of course includes act_mirred
execution, so why isn't it a problem there?

People added MIRRED_RECURSION_LIMIT for this kinda recursion,
but never released that root lock.

Thanks.