From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7A121C32796 for ; Wed, 24 Aug 2022 15:12:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238786AbiHXPME (ORCPT ); Wed, 24 Aug 2022 11:12:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40188 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238039AbiHXPMD (ORCPT ); Wed, 24 Aug 2022 11:12:03 -0400 Received: from out03.mta.xmission.com (out03.mta.xmission.com [166.70.13.233]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2945392F67; Wed, 24 Aug 2022 08:12:03 -0700 (PDT) Received: from in01.mta.xmission.com ([166.70.13.51]:46614) by out03.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1oQs2y-00CHHk-TK; Wed, 24 Aug 2022 09:12:01 -0600 Received: from ip68-110-29-46.om.om.cox.net ([68.110.29.46]:36266 helo=email.froward.int.ebiederm.org.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1oQs2w-001mpu-Q3; Wed, 24 Aug 2022 09:12:00 -0600 From: "Eric W. Biederman" To: Jens Axboe Cc: Olivier Langlois , Pavel Begunkov , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, Alexander Viro , Oleg Nesterov , Linus Torvalds References: <192c9697e379bf084636a8213108be6c3b948d0b.camel@trillion01.com> <9692dbb420eef43a9775f425cb8f6f33c9ba2db9.camel@trillion01.com> <87h7i694ij.fsf_-_@disp2133> <1b519092-2ebf-3800-306d-c354c24a9ad1@gmail.com> <13250a8d-1a59-4b7b-92e4-1231d73cbdda@gmail.com> <878rw9u6fb.fsf@email.froward.int.ebiederm.org> <303f7772-eb31-5beb-2bd0-4278566591b0@gmail.com> <87ilsg13yz.fsf@email.froward.int.ebiederm.org> <8218f1a245d054c940e25142fd00a5f17238d078.camel@trillion01.com> <87y1wnrap0.fsf_-_@email.froward.int.ebiederm.org> <87mtd3rals.fsf_-_@email.froward.int.ebiederm.org> <61abfb5a517e0ee253b0dc7ba9cd32ebd558bcb0.camel@trillion01.com> <875yiisttu.fsf@email.froward.int.ebiederm.org> <654cb5de-a563-b812-a435-d9b435cee334@kernel.dk> Date: Wed, 24 Aug 2022 10:11:23 -0500 In-Reply-To: <654cb5de-a563-b812-a435-d9b435cee334@kernel.dk> (Jens Axboe's message of "Tue, 23 Aug 2022 12:27:06 -0600") Message-ID: <87lerdr810.fsf@email.froward.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1oQs2w-001mpu-Q3;;;mid=<87lerdr810.fsf@email.froward.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.110.29.46;;;frm=ebiederm@xmission.com;;;spf=softfail X-XM-AID: U2FsdGVkX1/zIi/Z3JzOwDc+HihlayO1DxAMK324r+c= X-SA-Exim-Connect-IP: 68.110.29.46 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH 2/2] coredump: Allow coredumps to pipes to work with io_uring X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org Jens Axboe writes: > On 8/23/22 12:22 PM, Eric W. Biederman wrote: >> Olivier Langlois 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