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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe