netdev - Re: rtnl_trylock() versus SCHED

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Message-ID: <b2cabeeb-f36d-189b-2ce2-1f9605af0063@cumulusnetworks.com>
Date:   Sun, 9 Aug 2020 17:12:41 +0300
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Hillf Danton <hdanton@...a.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Cc:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Network Development <netdev@...r.kernel.org>,
        Markus Elfring <Markus.Elfring@....de>
Subject: Re: rtnl_trylock() versus SCHED_FIFO lockup

On 09/08/2020 16:49, Hillf Danton wrote:
> 
> On Fri, 7 Aug 2020 08:03:32 -0700 Stephen Hemminger wrote:
>> On Fri, 7 Aug 2020 10:03:59 +0200
>> Rasmus Villemoes <rasmus.villemoes@...vas.dk> wrote:
>>
>>> On 07/08/2020 05.39, Stephen Hemminger wrote:
>>>> On Thu, 6 Aug 2020 12:46:43 +0300
>>>> Nikolay Aleksandrov <nikolay@...ulusnetworks.com> wrote:
>>>>   
>>>>> On 06/08/2020 12:17, Rasmus Villemoes wrote:  
>>>>>> On 06/08/2020 01.34, Stephen Hemminger wrote:    
>>>>>>> On Wed, 5 Aug 2020 16:25:23 +0200  
>>>
>>>>>
>>>>> Hi Rasmus,
>>>>> I haven't tested anything but git history (and some grepping) points to deadlocks when
>>>>> sysfs entries are being changed under rtnl.
>>>>> For example check: af38f2989572704a846a5577b5ab3b1e2885cbfb and 336ca57c3b4e2b58ea3273e6d978ab3dfa387b4c
>>>>> This is a common usage pattern throughout net/, the bridge is not the only case and there are more
>>>>> commits which talk about deadlocks.
>>>>> Again I haven't verified anything but it seems on device delete (w/ rtnl held) -> sysfs delete
>>>>> would wait for current readers, but current readers might be stuck waiting on rtnl and we can deadlock.
>>>>>  
>>>>
>>>> I was referring to AB BA lock inversion problems.  
>>>
>>> Ah, so lock inversion, not priority inversion.
> 
> Hi folks,
> 
> Is it likely that kworker helps work around that deadlock, by
> acquiring the rtnl lock in the case that the current fails to
> trylock it?
> 
> Hillf

You know it's a user writing to a file expecting config change, right?
There are numerous problems with deferring it (e.g. error handling).

Thanks,
 Nik