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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CC8F2CDB465 for ; Thu, 19 Oct 2023 13:53:36 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qtTSd-000632-4x; Thu, 19 Oct 2023 09:53:18 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qtTSZ-0005xy-GS; Thu, 19 Oct 2023 09:53:11 -0400 Received: from proxmox-new.maurer-it.com ([94.136.29.106]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qtTSW-00043d-Oi; Thu, 19 Oct 2023 09:53:11 -0400 Received: from proxmox-new.maurer-it.com (localhost.localdomain [127.0.0.1]) by proxmox-new.maurer-it.com (Proxmox) with ESMTP id 33C0E430CC; Thu, 19 Oct 2023 15:53:05 +0200 (CEST) Message-ID: Date: Thu, 19 Oct 2023 15:53:00 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: deadlock when using iothread during backup_clean() Content-Language: en-US To: Kevin Wolf Cc: Vladimir Sementsov-Ogievskiy , Paolo Bonzini , QEMU Developers , Hanna Reitz , John Snow , "open list:Network Block Dev..." , Thomas Lamprecht References: <44ff810b-8ec6-0f11-420a-6efa2c7c2475@proxmox.com> <2ca4eb06-75c3-7bd8-972b-b37af47743dc@yandex-team.ru> <71e3112d-3d3f-fd55-4099-6765d4f22205@proxmox.com> <76f9678d-ed92-418e-8d1e-c6dc55f83279@proxmox.com> <3e1c941b-2b76-4f85-88cb-4a2a968eb3e6@proxmox.com> From: Fiona Ebner In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=94.136.29.106; envelope-from=f.ebner@proxmox.com; helo=proxmox-new.maurer-it.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 19.10.23 um 14:14 schrieb Kevin Wolf: > Am 18.10.2023 um 11:42 hat Fiona Ebner geschrieben: >> Am 17.10.23 um 16:20 schrieb Kevin Wolf: >>> Am 17.10.2023 um 15:37 hat Fiona Ebner geschrieben: >>>> Am 17.10.23 um 14:12 schrieb Kevin Wolf: >>>>> Am 17.10.2023 um 12:18 hat Fiona Ebner geschrieben: >>>>>> I ran into similar issues now with mirror, (both deadlocks and stuck >>>>>> guest IO at other times), and interestingly, also during job start. >>>>>> >>>>>> Also had a backtrace similar to [0] once, so I took a closer look. >>>>>> Probably was obvious to others already, but for the record: >>>>>> >>>>>> 1. the graph is locked by the main thread >>>>>> 2. the iothread holds the AioContext lock >>>>>> 3. the main thread waits on the AioContext lock >>>>>> 4. the iothread waits for coroutine spawned by blk_is_available() >>>>> >>>>> Where does this blk_is_available() in the iothread come from? Having it >>>>> wait without dropping the AioContext lock sounds like something that >>>>> we'd want to avoid. Ideally, devices using iothreads shouldn't use >>>>> synchronous requests at all, but I think scsi-disk might have some of >>>>> them. >>>>> >>>> >>>> It's part of the request handling in virtio-scsi: >>>> >>>>> #0 0x00007ff7f5f55136 in __ppoll (fds=0x7ff7e40030c0, nfds=8, timeout=, sigmask=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:42 >>>>> #1 0x00005587132615ab in qemu_poll_ns (fds=0x7ff7e40030c0, nfds=8, timeout=-1) at ../util/qemu-timer.c:339 >>>>> #2 0x000055871323e8b1 in fdmon_poll_wait (ctx=0x55871598d5e0, ready_list=0x7ff7f288ebe0, timeout=-1) at ../util/fdmon-poll.c:79 >>>>> #3 0x000055871323e1ed in aio_poll (ctx=0x55871598d5e0, blocking=true) at ../util/aio-posix.c:670 >>>>> #4 0x0000558713089efa in bdrv_poll_co (s=0x7ff7f288ec90) at /home/febner/repos/qemu/block/block-gen.h:43 >>>>> #5 0x000055871308c362 in blk_is_available (blk=0x55871599e2f0) at block/block-gen.c:1426 >>>>> #6 0x0000558712f6843b in virtio_scsi_ctx_check (s=0x558716c049c0, d=0x55871581cd30) at ../hw/scsi/virtio-scsi.c:290 >>> >>> Oh... So essentially for an assertion. >>> >>> I wonder if the blk_is_available() check introduced in 2a2d69f490c is >>> even necessary any more, because BlockBackend has its own AioContext >>> now. And if blk_bs(blk) != NULL isn't what we actually want to check if >>> the check is necessary, because calling bdrv_is_inserted() doesn't seem >>> to have been intended. blk_bs() wouldn't have to poll. >>> >> >> Could virtio_scsi_hotunplug() be an issue with removing or modifying >> the check? There's a call there which sets the blk's AioContext to >> qemu_get_aio_context(). Or are we sure that the assert in >> virtio_scsi_ctx_check() can't be reached after that? > > I think that would be the kind of bug that the assertion tries to > catch, because then we would be sending requests to blk from a thread > that doesn't match its AioContext (which will be allowed soon, but not > quite yet). > > Before resetting the AioContext, virtio_scsi_hotunplug() calls > qdev_simple_device_unplug_cb(), which unrealizes the SCSI device. This > calls scsi_qdev_unrealize() -> scsi_device_purge_requests(), which in > turn drains blk and cancels all pending requests. So there should be > nothing left that could call into virtio_scsi_ctx_check() any more. > > The other argument is that after unrealize, virtio_scsi_device_get() > would return NULL anyway, so even if a request were still pending, it > would just fail instead of accessing the unplugged device. > Okay, sounds like a way to get around that deadlock issue then :) (...) >>> >>> What does the stuck I/O look like? Is it stuck in the backend, i.e. the >>> device started requests that never complete? Or stuck from the guest >>> perspective, i.e. the device never checks for new requests? >>> >> >> AFAICT, from the guest perspective. >> >>> I don't really have an idea immediately, we'd have to find out where the >>> stuck I/O stops being processed. >>> >> >> I've described it in an earlier mail in this thread: >> https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg01900.html >> >> Quoting from there: >> >>> After the IO was stuck in the guest, I used bdrv_next_all_states() to >>> iterate over the states and there's only the bdrv_raw and the >>> bdrv_host_device. For both, tracked_requests was empty. > > And bs->in_flight and blk->in_flight are 0, too? > Yes. And queued_requests in the BlockBackend is also empty. > Is anything quiesced? No. quiesce_counter is 0 for both BlockDriverState instances as well as for the BlockBackend. quiesced_parent is false for both parents (i.e. child_root for the bdrv_raw and child_of_bds for the bdrv_file (this time I used VirtIO SCSI, in the quote it was VirtIO block)). >>> What is also very interesting is that the IO isn't always dead >>> immediately. It can be that the fio command still runs with lower speed >>> for a while (sometimes even up to about a minute, but most often about >>> 10-15 seconds or so). During that time, I still can see calls to >>> virtio_scsi_handle_cmd() and blk_aio_write_entry(). Then they suddenly stop. >> >> Noting again that (at least for backup) it happens with both virtio-blk >> and virtio-scsi and with both aio=io_uring and aio=threads. I also tried >> different host kernels 5.15, 6.2 and 6.5 and guest kernels 5.10 and 6.1. > > You say "at least for backup". Did you see the bug for other job types, > too? > Yes, sorry. I meant to say that I only tested it for the backup canceling with all those configurations. I did run into the stuck guest IO issue in three different scenarios: canceling backup, canceling mirror and starting mirror: >>>>> Am 17.10.2023 um 12:18 hat Fiona Ebner geschrieben: >>>>>> I ran into similar issues now with mirror, (both deadlocks and stuck >>>>>> guest IO at other times), and interestingly, also during job start. But for mirror, I only used VirtIO SCSI and aio=io_uring and didn't test other configurations. Best Regards, Fiona