Subject: Re: [PATCH net] net, sched: fix soft lockup in tc_classify

From: Daniel Borkmann <[email-masked]>
Date: Wed, 21 Dec 2016 22:07:48 +0100

On 12/21/2016 09:47 PM, Cong Wang wrote:
> On Wed, Dec 21, 2016 at 12:02 PM, Daniel Borkmann <[email-masked]> wrote:
>> On 12/21/2016 08:10 PM, Cong Wang wrote:
>>> On Wed, Dec 21, 2016 at 10:51 AM, Cong Wang <[email-masked]>
>>> wrote:
>>>> On Wed, Dec 21, 2016 at 9:04 AM, Daniel Borkmann <[email-masked]>
>>>> wrote:
>>>>>
>>>>> What happens is that in tc_ctl_tfilter(), thread A allocates a new
>>>>> tp, initializes it, sets tp_created to 1, and calls into
>>>>> tp->ops->change()
>>>>> with it. In that classifier callback we had to unlock/lock the rtnl
>>>>> mutex and returned with -EAGAIN. One reason why we need to drop there
>>>>> is, for example, that we need to request an action module to be loaded.
>>>>
>>>> Excellent catch!
>>>>
>>>> But why do we have to replay the request here? Shouldn't we just return
>>>> EAGAIN to user-space and let user-space decide to retry or not?
>>>> Replaying is the root of the evil here.
>>>
>>> Answer myself: probably due to historical reasons, but still replaying
>>> inside such a big function is just error-prone, we should promote it
>>> out:
>>
>> Have no strong opinion, I guess it could be done as a simplification
>> for net-next, why not, along with moving out the netlink_ns_capable()
>> check or possibly other things after careful analysis that don't need
>> to be redone in that circumstance.
>
> It is only slightly bigger than your current one so could fit for -stable too.
> Also, it could fix all potential problems like this one. Let compiler do the
> work, not human. ;)

Ok, you mean for net. In that case I prefer the smaller sized fix to be
honest. It also covers everything from the point where we fetch the chain
via cops->tcf_chain() to the end of the function, which is where most of
the complexity resides, and only the two mentioned commits do the relock,
so as a fix I think it's fine as-is. As mentioned, if there's need to
refactor tc_ctl_tfilter() net-next would be better, imho.

Thanks,
Daniel