Re: [PATCH RFC v4 12/15] io_uring/cmd: let cmds to know about dying task

[Pavel Begunkov <asml.silence@gmail.com>]

On 11/5/24 23:02, Bernd Schubert wrote:
> On 11/5/24 02:08, Pavel Begunkov wrote:
>> On 11/4/24 22:15, Bernd Schubert wrote:
>>> On 11/4/24 01:28, Pavel Begunkov wrote:
>> ...
>>>> In general if you need to change something, either stick your
>>>> name, so that I know it might be a derivative, or reflect it in
>>>> the commit message, e.g.
>>>>
>>>> Signed-off-by: initial author
>>>> [Person 2: changed this and that]
>>>> Signed-off-by: person 2
>>>
>>> Oh sorry, for sure. I totally forgot to update the commit message.
>>>
>>> Somehow the initial version didn't trigger. I need to double check to
>>
>> "Didn't trigger" like in "kernel was still crashing"?
> 
> My initial problem was a crash in iov_iter_get_pages2() on process
> kill. And when I tested your initial patch IO_URING_F_TASK_DEAD didn't
> get set. Jens then asked to test with the version that I have in my
> branch and that worked fine. Although in the mean time I wonder if
> I made test mistake (like just fuse.ko reload instead of reboot with
> new kernel). Just fixed a couple of issues in my branch (basically
> ready for the next version send), will test the initial patch
> again as first thing in the morning.

I see. Please let know if it doesn't work, it's not specific
to fuse, if there is a problem it'd also affects other core
io_uring parts.

>> FWIW, the original version is how it's handled in several places
>> across io_uring, and the difference is a gap for !DEFER_TASKRUN
>> when a task_work is queued somewhere in between when a task is
>> started going through exit() but haven't got PF_EXITING set yet.
>> IOW, should be harder to hit.
>>
> 
> Does that mean that the test for PF_EXITING is racy and we cannot
> entirely rely on it?

No, the PF_EXITING check was fine, even though it'll look
different now for unrelated reasons. What I'm saying is that the
callback can get executed from the desired task, i.e.
req->task == current, but it can happen from a late exit(2)/etc.
path where the task is botched and likely doesn't have ->mm.

-- 
Pavel Begunkov