netdev - Re: [net/sched] Question: Locks for clearing ERR_PTR() value from idrinfo->action

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Date: Fri, 14 Jun 2024 09:13:07 -0300
From: Pedro Tammela <pctammela@...atatu.com>
To: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
 Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>,
 Jiri Pirko <jiri@...nulli.us>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Network Development
 <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [net/sched] Question: Locks for clearing ERR_PTR() value from
 idrinfo->action_idr ?

On 14/06/2024 01:00, Tetsuo Handa wrote:
> On 2024/06/14 11:47, Pedro Tammela wrote:
>> On 13/06/2024 21:58, Tetsuo Handa wrote:
>>>
>>> Is there a possibility that tcf_idr_check_alloc() is called without holding
>>> rtnl_mutex?
>>
>> There is, but not in the code path of this reproducer.
>>
>>> If yes, adding a sleep before "goto again;" would help. But if no,
>>> is this a sign that some path forgot to call tcf_idr_{cleanup,insert_many}() ?
>>
>> The reproducer is sending a new action message with 2 actions.
>> Actions are committed to the idr after processing in order to make them visible
>> together and after any errors are caught.
>>
>> The bug happens when the actions in the message refer to the same index. Since
>> the first processing succeeds, adding -EBUSY to the index, the second processing,
>> which references the same index, will loop forever.
>>
>> After the change to rely on RCU for this check, instead of the idr lock, the hangs
>> became more noticeable to syzbot since now it's hanging a system-wide lock.
> 
> Thank you for explanation. Then, what type of sleep do we want?

This patch should fix the bug: 
https://lore.kernel.org/netdev/20240613071021.471432-1-druth@chromium.org/