Subject: Re: Soft lockup in tc_classify

From: Daniel Borkmann <[email-masked]>
Date: Mon, 12 Dec 2016 14:28:43 +0100

Hi Shahar,

On 12/12/2016 10:43 AM, Shahar Klein wrote:
> Hi All,
>
> sorry for the spam, the first time was sent with html part and was rejected.
>
> We observed an issue where a classifier instance next member is pointing back to itself, causing a CPU soft lockup.
> We found it by running traffic on many udp connections and then adding a new flower rule using tc.
>
> We added a quick workaround to verify it:
>
> In tc_classify:
>
>          for (; tp; tp = rcu_dereference_bh(tp->next)) {
>                  int err;
> +               if (tp == tp->next)
> +                     RCU_INIT_POINTER(tp->next, NULL);
>
>
> We also had a print here showing tp->next is pointing to tp. With this workaround we are not hitting the issue anymore.
> We are not sure we fully understand the mechanism here - with the rtnl and rcu locks.
> We'll appreciate your help solving this issue.

Note that there's still the RCU fix missing for the deletion race that
Cong will still send out, but you say that the only thing you do is to
add a single rule, but no other operation in involved during that test?

Do you have a script and kernel .config for reproducing this?

Thanks,
Daniel