Re: [GIT PULL] io_uring fixes for 5.15-rc3

[ebiederm@xmission.com (Eric W. Biederman)]

Jens Axboe <axboe@kernel.dk> writes:

> On 9/27/21 9:13 AM, Eric W. Biederman wrote:
>> Jens Axboe <axboe@kernel.dk> writes:
>> 
>>> On 9/27/21 8:29 AM, Jens Axboe wrote:
>>>> On 9/27/21 7:51 AM, Eric W. Biederman wrote:
>>>>> Jens Axboe <axboe@kernel.dk> writes:
>>>>>
>>>>>> On 9/25/21 5:05 PM, Linus Torvalds wrote:
>>>>>>> On Sat, Sep 25, 2021 at 1:32 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>>>
>>>>>>>> - io-wq core dump exit fix (me)
>>>>>>>
>>>>>>> Hmm.
>>>>>>>
>>>>>>> That one strikes me as odd.
>>>>>>>
>>>>>>> I get the feeling that if the io_uring thread needs to have that
>>>>>>> signal_group_exit() test, something is wrong in signal-land.
>>>>>>>
>>>>>>> It's basically a "fatal signal has been sent to another thread", and I
>>>>>>> really get the feeling that "fatal_signal_pending()" should just be
>>>>>>> modified to handle that case too.
>>>>>>
>>>>>> It did surprise me as well, which is why that previous change ended up
>>>>>> being broken for the coredump case... You could argue that the io-wq
>>>>>> thread should just exit on signal_pending(), which is what we did
>>>>>> before, but that really ends up sucking for workloads that do use
>>>>>> signals for communication purposes. postgres was the reporter here.
>>>>>
>>>>> The primary function get_signal is to make signals not pending.  So I
>>>>> don't understand any use of testing signal_pending after a call to
>>>>> get_signal.
>>>>>
>>>>> My confusion doubles when I consider the fact io_uring threads should
>>>>> only be dequeuing SIGSTOP and SIGKILL.
>>>>>
>>>>> I am concerned that an io_uring thread that dequeues SIGKILL won't call
>>>>> signal_group_exit and thus kill the other threads in the thread group.
>>>>>
>>>>> What motivated removing the break and adding the fatal_signal_pending
>>>>> test?
>>>>
>>>> I played with this a bit this morning, and I agree it doesn't seem to be
>>>> needed at all. The original issue was with postgres, I'll give that a
>>>> whirl as well and see if we run into any unwarranted exits. My simpler
>>>> test case did not.
>>>
>>> Ran the postgres test, and we get tons of io-wq exiting on get_signal()
>>> returning true. Took a closer look, and it actually looks very much
>>> expected, as it's a SIGKILL to the original task.
>>>
>>> So it looks like I was indeed wrong, and this probably masked the
>>> original issue that was fixed in that series. I've been running with
>>> this:
>>>
>>> diff --git a/fs/io-wq.c b/fs/io-wq.c
>>> index c2360cdc403d..afd1db8e000d 100644
>>> --- a/fs/io-wq.c
>>> +++ b/fs/io-wq.c
>>> @@ -584,10 +584,9 @@ static int io_wqe_worker(void *data)
>>>  
>>>  			if (!get_signal(&ksig))
>>>  				continue;
>>> -			if (fatal_signal_pending(current) ||
>>> -			    signal_group_exit(current->signal))
>>> -				break;
>>> -			continue;
>>> +			if (ksig.sig != SIGKILL)
>>> +				printk("exit on sig! fatal? %d, sig=%d\n", fatal_signal_pending(current), ksig.sig);
>>> +			break;
>>>  		}
>>>  		last_timeout = !ret;
>>>  	}
>>>
>>> and it's running fine and, as expected, we don't generate any printk
>>> activity as these are all fatal deliveries to the parent.
>> 
>> Good.  So just a break should be fine.
>
> Indeed, I'll send out a patch for that.
>
>> A little bit of me is concerned about not calling do_group_exit in this
>> case.  Fortunately it is not a problem as complete_signal kills all of
>> the threads in a signal_group when SIGKILL is delivered.
>> 
>> So at least until something else is refactored and io_uring threads
>> unblock another fatal signal all is well.
>
> Should we put a comment in io-wq to that effect? I don't see why we'd
> ever unblock other signals there, but...

I suspect rather we should update this comment in get_signal
instead.

		/*
		 * 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.
		 */
		if (current->flags & PF_IO_WORKER)
			goto out;


Although I would not mind updating io-wq.c and io_uring.c where
they call get_signal as well. 

Eric