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 F2057CDB474 for ; Tue, 17 Oct 2023 14:20:40 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qskvj-0007X6-5O; Tue, 17 Oct 2023 10:20:19 -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 1qskvh-0007Wq-KA for qemu-devel@nongnu.org; Tue, 17 Oct 2023 10:20:17 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qskvf-00067r-M6 for qemu-devel@nongnu.org; Tue, 17 Oct 2023 10:20:17 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1697552412; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=CGeHSDaNqv4AJ3TV5xUGDT+SOhBvDPy3xGnj2SBwXWM=; b=iHmC8FYeGe0A2A3PqyEWzvVl6VITz13f4xm5lfM9Xh6uoDvNdxXfV+2881TjkBxCCydMkN fy/PqCvrrzCgU9ZB0FwKmVMUtWaVaoFTVV9I8vSwDZDAVZglB1x3XCvOUJX5Pa3+mQAQxj V2xHoE67soufMTI9I0ejCG7iWrrZ+tU= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-212-U3XQxpZwMEuy2RzOYuUf2w-1; Tue, 17 Oct 2023 10:20:09 -0400 X-MC-Unique: U3XQxpZwMEuy2RzOYuUf2w-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id D94203827970; Tue, 17 Oct 2023 14:20:08 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.150]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 879941121314; Tue, 17 Oct 2023 14:20:07 +0000 (UTC) Date: Tue, 17 Oct 2023 16:20:06 +0200 From: Kevin Wolf To: Fiona Ebner Cc: Vladimir Sementsov-Ogievskiy , Paolo Bonzini , QEMU Developers , Hanna Reitz , John Snow , "open list:Network Block Dev..." , Thomas Lamprecht Subject: Re: deadlock when using iothread during backup_clean() Message-ID: 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, 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 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. > > #7 0x0000558712f697e4 in virtio_scsi_handle_cmd_req_prepare (s=0x558716c049c0, req=0x7ff7e400b650) at ../hw/scsi/virtio-scsi.c:788 > > #8 0x0000558712f699b0 in virtio_scsi_handle_cmd_vq (s=0x558716c049c0, vq=0x558716c0d2a8) at ../hw/scsi/virtio-scsi.c:831 > > #9 0x0000558712f69bcb in virtio_scsi_handle_cmd (vdev=0x558716c049c0, vq=0x558716c0d2a8) at ../hw/scsi/virtio-scsi.c:867 > > #10 0x0000558712f96812 in virtio_queue_notify_vq (vq=0x558716c0d2a8) at ../hw/virtio/virtio.c:2263 > > #11 0x0000558712f99b75 in virtio_queue_host_notifier_read (n=0x558716c0d31c) at ../hw/virtio/virtio.c:3575 > > #12 0x000055871323d8b5 in aio_dispatch_handler (ctx=0x55871598d5e0, node=0x558716771000) at ../util/aio-posix.c:372 > > #13 0x000055871323d988 in aio_dispatch_ready_handlers (ctx=0x55871598d5e0, ready_list=0x7ff7f288eeb0) at ../util/aio-posix.c:401 > > > >> As for why it doesn't progress, blk_co_is_available_entry() uses > >> bdrv_graph_co_rdlock() and can't get it, because the main thread has the > >> write lock. Should be fixed once the AioContext locks are gone, but not > >> sure what should be done to avoid it until then. > > > > Then the nested event loop in blk_is_available() would probably be > > enough to make progress, yes. > > > > Maybe we could actually drop the lock (and immediately reacquire it) in > > AIO_WAIT_WHILE() even if we're in the home thread? That should give the > > main thread a chance to make progress. > > Seems to work :) I haven't run into the issue with the following change > anymore, but I have to say, running into that specific deadlock only > happened every 10-15 tries or so before. Did 30 tests now. But > unfortunately, the stuck IO issue is still there. > > > diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h > > index 5449b6d742..da159501ca 100644 > > --- a/include/block/aio-wait.h > > +++ b/include/block/aio-wait.h > > @@ -88,7 +88,13 @@ extern AioWait global_aio_wait; > > smp_mb__after_rmw(); \ > > if (ctx_ && in_aio_context_home_thread(ctx_)) { \ > > while ((cond)) { \ > > + if (unlock && ctx_) { \ > > + aio_context_release(ctx_); \ > > + } \ > > aio_poll(ctx_, true); \ > > + if (unlock && ctx_) { \ > > + aio_context_acquire(ctx_); \ > > + } \ > > waited_ = true; \ > > } \ > > } else { \ For reacquiring the lock, I really meant "immediately". Calling aio_poll() without the lock is wrong. 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? I don't really have an idea immediately, we'd have to find out where the stuck I/O stops being processed. > > But I think we're actually not very far from removing the AioContext > > lock, so if we can't find an easy fix in the meantime, waiting for that > > could be a realistic option. > > > > Glad to hear :) Do you think it will be in time for QEMU 8.2? Otherwise, > I'll go ahead and send what I have for fixing the deadlocks from this > mail thread in the following days. The stuck guest IO can happen even > without any of those changes (on current master, i.e. > ebca80bbdb5c1650e4b753a3d13b43634e7dfe05, at least when starting a > mirror job). Having it in 8.2 is certainly the plan, but plans don't always work out. If you have fixes that aren't too ugly, we can still apply them. Kevin