Re: [PATCH 0/6] Allow signals for IO threads

Jens Axboe
2021-03-26T14:56:20Z

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