netdev - Re: 5.10.4+ hang with 'rmmod nf

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Message-ID: <20210108061653.GB19605@breakpoint.cc>
Date:   Fri, 8 Jan 2021 07:16:53 +0100
From:   Florian Westphal <fw@...len.de>
To:     Ben Greear <greearb@...delatech.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: 5.10.4+ hang with 'rmmod nf_conntrack'

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).

> I'll see if it is reproducible and if so will try
> with lockdep enabled...

No idea, there was a regression in 5.6, but that was fixed by the time
5.7 was released.

Can't reproduce hangs with a script that injects a few dummy entries
and then removes the module:

added=0

add_and_rmmod()
{
        while [ $added -lt 1000 ]; do
                conntrack -I -s $(($RANDOM%256)).$(($RANDOM%256)).$(($RANDOM%256)).$(($RANDOM%255+1)) \
                        -d $(($RANDOM%256)).$(($RANDOM%256)).$(($RANDOM%256)).$(($RANDOM%255+1)) \
                         --protonum 6 --timeout $(((RANDOM%120) + 240)) --state ESTABLISHED --sport $RANDOM --dport $RANDOM 2> /dev/null || break

                added=$((added + 1))
                if [ $((added % 1000)) -eq 0 ];then
                        echo $added
                fi
        done

        echo rmmod after adding $added entries
        conntrack -C
        rmmod nf_conntrack_netlink
        rmmod nf_conntrack
}

add_and_rmmod

I don't see how it would make a difference, but do you have any special conntrack features enabled
at run time, e.g. reliable netlink events? (If you don't know what I mean the answer is no).