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Subject: Re: [PATCH 0/6] Allow signals for IO threads
From:   Jens Axboe <axboe@kernel.dk>
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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Message-ID: <1304f480-a8db-44cf-5d89-aa9b99efdcd7@kernel.dk>
Date:   Fri, 26 Mar 2021 08:55:43 -0600
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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);

-- 
Jens Axboe