netdev - Re: 5.10.4+ hang with 'rmmod nf

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Message-ID: <d60c4cf1-8812-01e1-a51d-f1ada3c1b3fa@candelatech.com>
Date:   Fri, 22 Apr 2022 14:09:02 -0700
From:   Ben Greear <greearb@...delatech.com>
To:     Florian Westphal <fw@...len.de>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: 5.10.4+ hang with 'rmmod nf_conntrack'

On 4/22/22 9:32 AM, Ben Greear wrote:
> On 1/8/21 5:07 AM, Ben Greear wrote:
>> On 1/7/21 10:16 PM, Florian Westphal wrote:
>>> Ben Greear <greearb@...delatech.com> wrote:
>>>> I noticed my system has a hung process trying to 'rmmod nf_conntrack'.
>>>>
>>>> I've generally been doing the script that calls rmmod forever,
>>>> but only extensively tested on 5.4 kernel and earlier.
>>>>
>>>> If anyone has any ideas, please let me know.  This is from 'sysrq t'.  I don't see
>>>> any hung-task splats in dmesg.
>>>
>>> rmmod on conntrack loops forever until the active conntrack object count reaches 0.
>>> (plus a walk of the conntrack table to evict/put all entries).
> 
> Hello Florian,
> 
> I keep hitting this bug in a particular test case in 5.17.4+, so I added some debug to
> try to learn more.
> 
> My debugging patch looks like this:
> 
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index 7552e1e9fd62..29724114caef 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -2543,6 +2543,7 @@ void nf_conntrack_cleanup_net_list(struct list_head *net_exit_list)
>   {
>          int busy;
>          struct net *net;
> +       unsigned long loops = 0;
> 
>          /*
>           * This makes sure all current packets have passed through
> @@ -2556,12 +2557,30 @@ void nf_conntrack_cleanup_net_list(struct list_head *net_exit_list)
>                  struct nf_conntrack_net *cnet = nf_ct_pernet(net);
> 
>                  nf_ct_iterate_cleanup(kill_all, net, 0, 0);
> -               if (atomic_read(&cnet->count) != 0)
> +               if (atomic_read(&cnet->count) != 0) {
> +                       if (loops > 50010)
> +                               pr_err("nf-conntrack-cleanup-net-list, loops: %ld  cnet-count: %d, expect-count: %d users4: %d users6: %d  users_bridge: %d\n",
> +                                      loops, atomic_read(&cnet->count), cnet->expect_count,
> +                                      cnet->users4, cnet->users6, cnet->users_bridge);
>                          busy = 1;
> +               }
>          }
>          if (busy) {
> +               loops++;
> +               if (loops > 50000) {
> +                       msleep(500);
> +               }
>                  schedule();
> -               goto i_see_dead_people;
> +               if (loops > 50020) {
> +                       /* This thing is wedged, going to require a reboot to recover, so attempt
> +                        * to just ignore the bad count and see if system works OK.
> +                        */
> +                       WARN_ON_ONCE(1);
> +                       pr_err("ERROR:  nf_conntrack_cleanup_net cannot make progress.  Ignoring stale reference count and will continue.\n");
> +               }
> +               else {
> +                       goto i_see_dead_people;
> +               }
>          }
> 
>          list_for_each_entry(net, net_exit_list, exit_list) {
> 
> 
> Do you (or anyone else), have some ideas for how to debug this further to help find where the reference
> is leaked (or not released)?

I am now quite sure that the problem I was seeing was caused by an skb leak in the mt76 driver
(for which Felix just found a solution).  After that fix, then I no longer see the nf_conntrack
rmmod hangs.  I will keep testing in case I am just geting (un)lucky.

I do plan to keep my hack/patch in my kernel though, I'd rather it continue and leak some more
memory instead of busy hang forever when skb leaks are hit...

Thanks,
Ben