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Subject: Re: [PATCH 0/6] Allow signals for IO threads
To:     Stefan Metzmacher <metze@samba.org>, io-uring@vger.kernel.org
Cc:     torvalds@linux-foundation.org, ebiederm@xmission.com,
        oleg@redhat.com, linux-kernel@vger.kernel.org
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <26b06a25-9be9-102e-09f5-b2cb7c0fbc5e@kernel.dk>
Date:   Fri, 26 Mar 2021 09:12:16 -0600
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On 3/26/21 9:11 AM, Stefan Metzmacher wrote:
> Am 26.03.21 um 16:10 schrieb Jens Axboe:
>> On 3/26/21 9:08 AM, Stefan Metzmacher wrote:
>>> Am 26.03.21 um 15:55 schrieb Jens Axboe:
>>>> On 3/26/21 8:53 AM, Jens Axboe wrote:
>>>>> On 3/26/21 8:45 AM, Stefan Metzmacher wrote:
>>>>>> Am 26.03.21 um 15:43 schrieb Stefan Metzmacher:
>>>>>>> Am 26.03.21 um 15:38 schrieb Jens Axboe:
>>>>>>>> On 3/26/21 7:59 AM, Jens Axboe wrote:
>>>>>>>>> On 3/26/21 7:54 AM, Jens Axboe wrote:
>>>>>>>>>>> The KILL after STOP deadlock still exists.
>>>>>>>>>>
>>>>>>>>>> In which tree? Sounds like you're still on the old one with that
>>>>>>>>>> incremental you sent, which wasn't complete.
>>>>>>>>>>
>>>>>>>>>>> Does io_wq_manager() exits without cleaning up on SIGKILL?
>>>>>>>>>>
>>>>>>>>>> No, it should kill up in all cases. I'll try your stop + kill, I just
>>>>>>>>>> tested both of them separately and didn't observe anything. I also ran
>>>>>>>>>> your io_uring-cp example (and found a bug in the example, fixed and
>>>>>>>>>> pushed), fwiw.
>>>>>>>>>
>>>>>>>>> I can reproduce this one! I'll take a closer look.
>>>>>>>>
>>>>>>>> OK, that one is actually pretty straight forward - we rely on cleaning
>>>>>>>> up on exit, but for fatal cases, get_signal() will call do_exit() for us
>>>>>>>> and never return. So we might need a special case in there to deal with
>>>>>>>> that, or some other way of ensuring that fatal signal gets processed
>>>>>>>> correctly for IO threads.
>>>>>>>
>>>>>>> And if (fatal_signal_pending(current)) doesn't prevent get_signal() from being called?
>>>>>>
>>>>>> Ah, we're still in the first get_signal() from SIGSTOP, correct?
>>>>>
>>>>> Yes exactly, we're waiting in there being stopped. So we either need to
>>>>> check to something ala:
>>>>>
>>>>> relock:
>>>>> +	if (current->flags & PF_IO_WORKER && fatal_signal_pending(current))
>>>>> +		return false;
>>>>>
>>>>> to catch it upfront and from the relock case, or add:
>>>>>
>>>>> 	fatal:
>>>>> +		if (current->flags & PF_IO_WORKER)
>>>>> +			return false;
>>>>>
>>>>> to catch it in the fatal section.
>>>>
>>>> Can you try this? Not crazy about adding a special case, but I don't
>>>> think there's any way around this one. And should be pretty cheap, as
>>>> we're already pulling in ->flags right above anyway.
>>>>
>>>> diff --git a/kernel/signal.c b/kernel/signal.c
>>>> index 5ad8566534e7..5b75fbe3d2d6 100644
>>>> --- a/kernel/signal.c
>>>> +++ b/kernel/signal.c
>>>> @@ -2752,6 +2752,15 @@ bool get_signal(struct ksignal *ksig)
>>>>  		 */
>>>>  		current->flags |= PF_SIGNALED;
>>>>  
>>>> +		/*
>>>> +		 * PF_IO_WORKER threads will catch and exit on fatal signals
>>>> +		 * themselves. They have cleanup that must be performed, so
>>>> +		 * we cannot call do_exit() on their behalf. coredumps also
>>>> +		 * do not apply to them.
>>>> +		 */
>>>> +		if (current->flags & PF_IO_WORKER)
>>>> +			return false;
>>>> +
>>>>  		if (sig_kernel_coredump(signr)) {
>>>>  			if (print_fatal_signals)
>>>>  				print_fatal_signal(ksig->info.si_signo);
>>>>
>>>
>>> I guess not before next week, but if it resolves the problem for you,
>>> I guess it would be good to get this into rc5.
>>
>> It does, I pushed out a new branch. I'll send out a v2 series in a bit.
> 
> Great, thanks!
> 
> Any chance to get the "cmdline" hiding included?

I'll take a look at your response there, haven't yet. Wanted to get this
one sorted first.

-- 
Jens Axboe