Subject: Re: [PATCH net-next 1/2] net/sched: act_api: rely on rcu in tcf_idr_check_alloc

From: Vlad Buslov <[email-masked]>
Date: Mon, 11 Dec 2023 18:10:28 +0200

On Fri 08 Dec 2023 at 18:07, Pedro Tammela <[email-masked]> wrote:
> On 06/12/2023 06:52, Vlad Buslov wrote:
>>> Ok, so if I'm binding and it's observed a free index, which means "try to
>>> allocate" and I get a ENOSPC after jumping to new, try again but this time
>>> binding into the allocated action.
>>>
>>> In this scenario when we come back to 'again' we will wait until -EBUSY is
>>> replaced with the real pointer. Seems like a big enough window that any race for
>>> allocating from binding would most probably end up in this contention loop.
>>>
>>> However I think when we have these two retry mechanisms there's a extremely
>>> small window for an infinite loop if an action delete is timed just right, in
>>> between the action pointer is found and when we grab the tcfa_refcnt.
>>>
>>> 	idr_find (pointer)
>>> 	tcfa_refcnt (0)  <-------|
>>> 	again:                   |
>>> 	idr_find (free index!)   |
>>> 	new:                     |
>>> 	idr_alloc_u32 (ENOSPC)   |
>>> 	again:                   |
>>> 	idr_find (EBUSY)         |
>>> 	again:                   |
>>> 	idr_find (pointer)       |
>>> 	<evil delete happens>    |
>>> 	------->>>>--------------|
>> I'm not sure I'm following. Why would this sequence cause infinite loop?
>> 
>
> Perhaps I was being overly paranoid. Taking a look again it seems that not only
> an evil delete but also EBUSY must be in the action idr for a long time. I see
> it now, it looks like it converges.
>
> I was wondering if instead of looping in 'again:' in either scenarios you
> presented, what if we return -EAGAIN and let the filter infrastructure retry it
> under rtnl_lock()? At least will give enough breathing room for a call to
> schedule() to kick in if needed.

Sounds good, but you will need to ensure that both act and cls api
implementations properly retry on EAGAIN (looks like they do, but I only
gave it a cursory glance).