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 40295C636D4 for ; Wed, 15 Feb 2023 18:19:59 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pSMNh-00067a-3a; Wed, 15 Feb 2023 13:19:49 -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 1pSMNf-00067G-Uz for qemu-devel@nongnu.org; Wed, 15 Feb 2023 13:19:47 -0500 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 1pSMNe-0003he-HN for qemu-devel@nongnu.org; Wed, 15 Feb 2023 13:19:47 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1676485185; 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=jX0rUjyqJGqxzoh5gc4lX6Yp69D+xyH+YQ/g8AkaiKE=; b=DPqO5ZaqLmHZVBS8a4N1LR4gD+4ACpDLKq72f0lqz+AZ2GVm5k6924Y2IzWf2KBgJIA1Aq mmRf5VBi4HpcRECDICn9o4oU4RFPkPzHsgBpN5/rXX5T17ohYjlw0OHMuk1D3CxudwAlgR Azh7vpyPnZeZFQGnb+yun9I2o4cuqUw= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-568-tRpz02diNIO58Bwl8snCDg-1; Wed, 15 Feb 2023 13:19:42 -0500 X-MC-Unique: tRpz02diNIO58Bwl8snCDg-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 2EF7785CBE7; Wed, 15 Feb 2023 18:19:41 +0000 (UTC) Received: from redhat.com (unknown [10.2.16.217]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5B5FD492B0E; Wed, 15 Feb 2023 18:19:40 +0000 (UTC) Date: Wed, 15 Feb 2023 12:19:38 -0600 From: Eric Blake To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, "Michael S. Tsirkin" , Paolo Bonzini , David Hildenbrand , Peter Xu , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Fam Zheng Subject: Re: [PATCH v2 2/3] dma-helpers: prevent dma_blk_cb() vs dma_aio_cancel() race Message-ID: <20230215181938.s45ilx7jrvpwl4ie@redhat.com> References: <20230210143238.524357-1-stefanha@redhat.com> <20230210143238.524357-3-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230210143238.524357-3-stefanha@redhat.com> User-Agent: NeoMutt/20220429 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.9 Received-SPF: pass client-ip=170.10.133.124; envelope-from=eblake@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_H2=-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 On Fri, Feb 10, 2023 at 09:32:37AM -0500, Stefan Hajnoczi wrote: > dma_blk_cb() only takes the AioContext lock around ->io_func(). That > means the rest of dma_blk_cb() is not protected. In particular, the > DMAAIOCB field accesses happen outside the lock. > > There is a race when the main loop thread holds the AioContext lock and > invokes scsi_device_purge_requests() -> bdrv_aio_cancel() -> > dma_aio_cancel() while an IOThread executes dma_blk_cb(). The dbs->acb > field determines how cancellation proceeds. If dma_aio_cancel() see "sees" or "can see" > dbs->acb == NULL while dma_blk_cb() is still running, the request can be > completed twice (-ECANCELED and the actual return value). > > The following assertion can occur with virtio-scsi when an IOThread is > used: > > ../hw/scsi/scsi-disk.c:368: scsi_dma_complete: Assertion `r->req.aiocb != NULL' failed. > > Fix the race by holding the AioContext across dma_blk_cb(). Now > dma_aio_cancel() under the AioContext lock will not see > inconsistent/intermediate states. > > Cc: Paolo Bonzini > Signed-off-by: Stefan Hajnoczi > --- > hw/scsi/scsi-disk.c | 4 +--- > softmmu/dma-helpers.c | 12 +++++++----- > 2 files changed, 8 insertions(+), 8 deletions(-) Widening the scope protected by the lock makes sense, insofar as we still need the lock. Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org