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 F3F53C47258 for ; Tue, 23 Jan 2024 17:11:09 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rSKIV-0005TP-JB; Tue, 23 Jan 2024 12:10:56 -0500 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 1rSKIG-0005Qu-PD for qemu-devel@nongnu.org; Tue, 23 Jan 2024 12:10:37 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rSKIE-000138-WB for qemu-devel@nongnu.org; Tue, 23 Jan 2024 12:10:36 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1706029834; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=BAkp+BYRjTKdRDDS4zCrFdicWn15E6/a/FmX1uJXkEI=; b=KhBVxxVuUIMwRRDMKbKzadJUtccDJ3XEASYB+TzdOHij381usF/90w6FVvX86NmnWgxoDE U3FeGz8O+Bt7HUf3USXr5jgX704nlz1QXwX/uyZmDU0HPTuvL83kem1WZrv41oalu1UsVi PCbkpTWQVrho0T/geLvFldNCp0L7I+g= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-437-7ubxB0SBMWCXFKSXvsHGOg-1; Tue, 23 Jan 2024 12:10:31 -0500 X-MC-Unique: 7ubxB0SBMWCXFKSXvsHGOg-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 16A60185A7A8; Tue, 23 Jan 2024 17:10:28 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.232]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 578812905; Tue, 23 Jan 2024 17:10:27 +0000 (UTC) Date: Tue, 23 Jan 2024 18:10:26 +0100 From: Kevin Wolf To: Hanna Czenczek Cc: qemu-block@nongnu.org, stefanha@redhat.com, qemu-devel@nongnu.org Subject: Re: [PULL 11/33] scsi: only access SCSIDevice->requests from one thread Message-ID: References: <20231221212339.164439-1-kwolf@redhat.com> <20231221212339.164439-12-kwolf@redhat.com> <73e752b2-a037-4b10-a903-56fa6ad75c6e@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <73e752b2-a037-4b10-a903-56fa6ad75c6e@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.327, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable 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 23.01.2024 um 17:40 hat Hanna Czenczek geschrieben: > On 21.12.23 22:23, Kevin Wolf wrote: > > From: Stefan Hajnoczi > > > > Stop depending on the AioContext lock and instead access > > SCSIDevice->requests from only one thread at a time: > > - When the VM is running only the BlockBackend's AioContext may access > > the requests list. > > - When the VM is stopped only the main loop may access the requests > > list. > > > > These constraints protect the requests list without the need for locking > > in the I/O code path. > > > > Note that multiple IOThreads are not supported yet because the code > > assumes all SCSIRequests are executed from a single AioContext. Leave > > that as future work. > > > > Signed-off-by: Stefan Hajnoczi > > Reviewed-by: Eric Blake > > Message-ID:<20231204164259.1515217-2-stefanha@redhat.com> > > Signed-off-by: Kevin Wolf > > --- > > include/hw/scsi/scsi.h | 7 +- > > hw/scsi/scsi-bus.c | 181 ++++++++++++++++++++++++++++------------- > > 2 files changed, 131 insertions(+), 57 deletions(-) > > My reproducer for https://issues.redhat.com/browse/RHEL-3934 now breaks more > often because of this commit than because of the original bug, i.e. when > repeatedly hot-plugging and unplugging a virtio-scsi and a scsi-hd device, > this tends to happen when unplugging the scsi-hd: > > {"execute":"device_del","arguments":{"id":"stg0"}} > {"return": {}} > qemu-system-x86_64: ../block/block-backend.c:2429: blk_get_aio_context: > Assertion `ctx == blk->ctx' failed. > > (gdb) bt > #0  0x00007f32a668d83c in  () at /usr/lib/libc.so.6 > #1  0x00007f32a663d668 in raise () at /usr/lib/libc.so.6 > #2  0x00007f32a66254b8 in abort () at /usr/lib/libc.so.6 > #3  0x00007f32a66253dc in  () at /usr/lib/libc.so.6 > #4  0x00007f32a6635d26 in  () at /usr/lib/libc.so.6 > #5  0x0000556e6b4880a4 in blk_get_aio_context (blk=0x556e6e89ccf0) at > ../block/block-backend.c:2429 > #6  blk_get_aio_context (blk=0x556e6e89ccf0) at > ../block/block-backend.c:2417 > #7  0x0000556e6b112d87 in scsi_device_for_each_req_async_bh > (opaque=0x556e6e2c6d10) at ../hw/scsi/scsi-bus.c:128 > #8  0x0000556e6b5d1966 in aio_bh_poll (ctx=ctx@entry=0x556e6d8aa290) at > ../util/async.c:218 > #9  0x0000556e6b5bb16a in aio_poll (ctx=0x556e6d8aa290, > blocking=blocking@entry=true) at ../util/aio-posix.c:722 > #10 0x0000556e6b4564b6 in iothread_run (opaque=opaque@entry=0x556e6d89d920) > at ../iothread.c:63 > #11 0x0000556e6b5bde58 in qemu_thread_start (args=0x556e6d8aa9b0) at > ../util/qemu-thread-posix.c:541 > #12 0x00007f32a668b9eb in  () at /usr/lib/libc.so.6 > #13 0x00007f32a670f7cc in  () at /usr/lib/libc.so.6 > > I don’t know anything about the problem yet, but as usual, I like > speculation and discovering how wrong I was later on, so one thing I came > across that’s funny about virtio-scsi is that requests can happen even while > a disk is being attached or detached.  That is, Linux seems to probe all > LUNs when a new virtio-scsi device is being attached, and it won’t stop just > because a disk is being attached or removed.  So maybe that’s part of the > problem, that we get a request while the BB is being detached, and > temporarily in an inconsistent state (BDS context differs from BB context). I don't know anything about the problem either, but since you already speculated about the cause, let me speculate about the solution: Can we hold the graph writer lock for the tran_commit() call in bdrv_try_change_aio_context()? And of course take the reader lock for blk_get_aio_context(), but that should be completely unproblematic. At the first sight I don't see a reason why this would break something, but I've learnt not to trust my first impression with the graph locking work... Of course, I also didn't check if there are more things inside of the device emulation that need additional locking in this case, too. But even if so, blk_get_aio_context() should never see an inconsistent state. Kevin