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From: "Eric W. Biederman" <[email protected]>
To: Jens Axboe <[email protected]>
Cc: Olivier Langlois <[email protected]>,
	Pavel Begunkov <[email protected]>,
	[email protected], [email protected],
	[email protected],
	Alexander Viro <[email protected]>,
	Oleg Nesterov <[email protected]>,
	Linus Torvalds <[email protected]>
Subject: Re: [PATCH 2/2] coredump: Allow coredumps to pipes to work with io_uring
Date: Wed, 24 Aug 2022 10:11:23 -0500	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]> (Jens Axboe's message of "Tue, 23 Aug 2022 12:27:06 -0600")

Jens Axboe <[email protected]> writes:

> On 8/23/22 12:22 PM, Eric W. Biederman wrote:
>> Olivier Langlois <[email protected]> writes:
>> 
>>> On Mon, 2022-08-22 at 17:16 -0400, Olivier Langlois wrote:
>>>>
>>>> What is stopping the task calling do_coredump() to be interrupted and
>>>> call task_work_add() from the interrupt context?
>>>>
>>>> This is precisely what I was experiencing last summer when I did work
>>>> on this issue.
>>>>
>>>> My understanding of how async I/O works with io_uring is that the
>>>> task
>>>> is added to a wait queue without being put to sleep and when the
>>>> io_uring callback is called from the interrupt context,
>>>> task_work_add()
>>>> is called so that the next time io_uring syscall is invoked, pending
>>>> work is processed to complete the I/O.
>>>>
>>>> So if:
>>>>
>>>> 1. io_uring request is initiated AND the task is in a wait queue
>>>> 2. do_coredump() is called before the I/O is completed
>>>>
>>>> IMHO, this is how you end up having task_work_add() called while the
>>>> coredump is generated.
>>>>
>>> I forgot to add that I have experienced the issue with TCP/IP I/O.
>>>
>>> I suspect that with a TCP socket, the race condition window is much
>>> larger than if it was disk I/O and this might make it easier to
>>> reproduce the issue this way...
>> 
>> I was under the apparently mistaken impression that the io_uring
>> task_work_add only comes from the io_uring userspace helper threads.
>> Those are definitely suppressed by my change.
>> 
>> Do you have any idea in the code where io_uring code is being called in
>> an interrupt context?  I would really like to trace that code path so I
>> have a better grasp on what is happening.
>> 
>> If task_work_add is being called from interrupt context then something
>> additional from what I have proposed certainly needs to be done.
>
> task_work may come from the helper threads, but generally it does not.
> One example would be doing a read from a socket. There's no data there,
> poll is armed to trigger a retry. When we get the poll notification that
> there's now data to be read, then we kick that off with task_work. Since
> it's from the poll handler, it can trigger from interrupt context. See
> the path from io_uring/poll.c:io_poll_wake() -> __io_poll_execute() ->
> io_req_task_work_add() -> task_work_add().

But that is a task_work to the helper thread correct?

> It can also happen for regular IRQ based reads from regular files, where
> the completion is actually done via task_work added from the potentially
> IRQ based completion path.

I can see that.

Which leaves me with the question do these task_work's directly wake up
the thread that submitted the I/O request?   Or is there likely to be
something for an I/O thread to do before an ordinary program thread is
notified.

I am asking because it is only the case of notifying ordinary program
threads that is interesting in the case of a coredump.  As I understand
it a data to read notification would typically be handled by the I/O
uring worker thread to trigger reading the data before letting userspace
know everything it asked to be done is complete.

Eric

  reply	other threads:[~2022-08-24 15:12 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <[email protected]>
     [not found] ` <[email protected]>
     [not found]   ` <87h7i694ij.fsf_-_@disp2133>
2021-06-09 20:33     ` [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL Linus Torvalds
2021-06-09 20:48       ` Eric W. Biederman
2021-06-09 20:52         ` Linus Torvalds
2021-06-09 21:02       ` Olivier Langlois
2021-06-09 21:05         ` Eric W. Biederman
2021-06-09 21:26           ` Olivier Langlois
2021-06-09 21:56             ` Olivier Langlois
2021-06-10 14:26             ` Eric W. Biederman
2021-06-10 15:17               ` Olivier Langlois
2021-06-10 18:58               ` [CFT}[PATCH] coredump: Limit what can interrupt coredumps Eric W. Biederman
2021-06-10 19:10                 ` Linus Torvalds
2021-06-10 19:18                   ` Eric W. Biederman
2021-06-10 19:50                     ` Linus Torvalds
2021-06-10 20:11                       ` [PATCH] " Eric W. Biederman
2021-06-10 21:04                         ` Linus Torvalds
2021-06-12 14:36                         ` Olivier Langlois
2021-06-12 16:26                           ` Jens Axboe
2021-06-14 14:10                         ` Oleg Nesterov
2021-06-14 16:37                           ` Eric W. Biederman
2021-06-14 16:59                             ` Oleg Nesterov
2021-06-15 22:08                           ` Eric W. Biederman
2021-06-16 19:23                             ` Olivier Langlois
2021-06-16 20:00                               ` Eric W. Biederman
2021-06-18 20:05                                 ` Olivier Langlois
2021-08-05 13:06                             ` Olivier Langlois
2021-08-10 21:48                               ` Tony Battersby
2021-08-11 20:47                                 ` Olivier Langlois
2021-08-12  1:55                                 ` Jens Axboe
2021-08-12 13:53                                   ` Tony Battersby
2021-08-15 20:42                                   ` Olivier Langlois
2021-08-16 13:02                                     ` Pavel Begunkov
2021-08-16 13:06                                       ` Pavel Begunkov
2021-08-17 18:15                                     ` Jens Axboe
2021-08-17 18:24                                       ` Jens Axboe
2021-08-17 19:29                                         ` Tony Battersby
2021-08-17 19:59                                           ` Jens Axboe
2021-08-17 21:28                                             ` Jens Axboe
2021-08-17 21:39                                               ` Tony Battersby
2021-08-17 22:05                                                 ` Jens Axboe
2021-08-18 14:37                                                   ` Tony Battersby
2021-08-18 14:46                                                     ` Jens Axboe
2021-08-18  2:57                                               ` Jens Axboe
2021-08-18  2:58                                                 ` Jens Axboe
2021-08-21 10:08                                                 ` Olivier Langlois
2021-08-21 16:47                                                   ` Olivier Langlois
2021-08-21 16:51                                                     ` Jens Axboe
2021-08-21 17:21                                                       ` Olivier Langlois
2021-08-21  9:52                                         ` Olivier Langlois
2021-08-21  9:48                                       ` Olivier Langlois
2021-10-22 14:13     ` [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL Pavel Begunkov
2021-12-24  1:34       ` Olivier Langlois
2021-12-24 10:37         ` Pavel Begunkov
2021-12-24 19:52           ` Eric W. Biederman
2021-12-28 11:24             ` Pavel Begunkov
2022-03-14 23:58               ` Eric W. Biederman
     [not found]                 ` <[email protected]>
2022-06-01  3:15                   ` Jens Axboe
2022-07-20 16:49                     ` [PATCH 0/2] coredump: Allow io_uring using apps to dump to pipes Eric W. Biederman
2022-07-20 16:50                       ` [PATCH 1/2] signal: Move stopping for the coredump from do_exit into get_signal Eric W. Biederman
2022-07-20 16:51                       ` [PATCH 2/2] coredump: Allow coredumps to pipes to work with io_uring Eric W. Biederman
2022-08-22 21:16                         ` Olivier Langlois
2022-08-23  3:35                           ` Olivier Langlois
2022-08-23 18:22                             ` Eric W. Biederman
2022-08-23 18:27                               ` Jens Axboe
2022-08-24 15:11                                 ` Eric W. Biederman [this message]
2022-08-24 15:51                                   ` Jens Axboe
2022-01-05 19:39           ` [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL Olivier Langlois

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