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Message-ID: <d1443c9d-f57d-427f-9517-89b8c9f8bbf1@mojatatu.com>
Date: Fri, 14 Jun 2024 09:13:07 -0300
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Subject: Re: [net/sched] Question: Locks for clearing ERR_PTR() value from
 idrinfo->action_idr ?
To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
 Jamal Hadi Salim <jhs@mojatatu.com>, Cong Wang <xiyou.wangcong@gmail.com>,
 Jiri Pirko <jiri@resnulli.us>
Cc: "David S. Miller" <davem@davemloft.net>,
 Eric Dumazet <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>,
 Paolo Abeni <pabeni@redhat.com>, Network Development
 <netdev@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
References: <8d61200a-a739-4200-a8a3-5386a834d44f@I-love.SAKURA.ne.jp>
 <de8e2709-8d7f-4e51-a4a4-35bad72ba136@mojatatu.com>
 <522c0b17-c515-475d-8224-637ca0eaf6a2@I-love.SAKURA.ne.jp>
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From: Pedro Tammela <pctammela@mojatatu.com>
In-Reply-To: <522c0b17-c515-475d-8224-637ca0eaf6a2@I-love.SAKURA.ne.jp>
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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/