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 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.

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.

Eric