Comment 10 for bug 1854968

Easy steps to reproduce this issue:

sudo modprobe l2tp_core
sudo ./linux-5.4.0/tools/testing/selftests/net/l2tp.sh
./close

where close is compiled from:

#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
        int fd;

        printf("calling socket..\n");
        fd = socket(AF_APPLETALK, SOCK_STREAM, 0);
        printf("socket returned: %d\n", fd);
}

When running the above program we just see "calling socket" and it blocks forever on the socket call. After a couple of minutes we get the kernel hung task warning. We also see repeated messages:

unregister_netdevice: waiting for eth0 to become free. Usage count = 1

I'll bisect the kernel next.