How to get the state of a "plug" qdisc ?

[MathisMorin]

The question seem simple, but I can't figure out how to get that. Here is my use case: I have two network namespaces (red at 192.168.15.1 and blue at 192.168.15.2) connected by a virtual ethernet cable. Red side, the device is named veth-red, blue side, it is named veth-blue. Now i want to add a "plug" qdisc to veth-blue, I'm doing it with the following command lines:

ip netns exec blue nl-qdisc-add --dev veth-blue --parent root --id 1: plug --limit 32000
ip netns exec blue nl-qdisc-add --dev veth-blue --parent root --id 1: --update plug --release-indefinite

Now, I want to be able to "buffer" and "release" my qdisc from a C program. My code looks like that:

#include <stdio.h>
#include <stdlib.h>
#include <libnl3/netlink/route/link.h>
#include <libnl3/netlink/route/tc.h>
#include <libnl3/netlink/route/qdisc.h>
#include <libnl3/netlink/route/qdisc/netem.h>
#include <libnl3/netlink/route/qdisc/tbf.h>

int main(){
    struct nl_sock *sock;
    struct nl_cache *cache;
    struct rtnl_link *link;
    int if_index;
    
    sock = nl_socket_alloc();
    nl_connect(sock, NETLINK_ROUTE);
    rtnl_link_alloc_cache(sock, AF_UNSPEC, &cache);
    link = rtnl_link_get_by_name(cache, "veth-blue");
    if_index = rtnl_link_get_ifindex(link);
    
    struct nl_cache *qdisc_cache;
    struct rtnl_qdisc *q;
    uint32_t handle = (1 << 16) | (0);

    rtnl_qdisc_alloc_cache(sock, &qdisc_cache);
    
    q = rtnl_qdisc_get(qdisc_cache, if_index, handle);
    rtnl_qdisc_plug_buffer(q);
    rtnl_qdisc_add(sock, q, NLM_F_CREATE);

    /* some code */

    rtnl_qdisc_plug_release_indefinite(q);
    rtnl_qdisc_add(sock, q, NLM_F_CREATE);

    return 0;
}

My code works fine, my only problem is that I want to be absolutely sure to execute the "/* some code */" part once the qdisc is "buffered" and not before. Thus, I'm looking for a indicator that could tell me whether or not the qdisc is buffered.

I looked into q with gdb, but can't find anything changing between the two states, I tried with rtnl_tc_data(TC_CAST(q)), but it seems it doesn't help me finding what I'm looking for.

Thanks in advance.

[chengyechun]

You can use the rtnl_qdisc_get_by_parent function or rtnl_qdisc_get to query.