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 995E9C54F30 for ; Tue, 27 May 2025 15:13:01 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uJvyW-0003f4-RV; Tue, 27 May 2025 11:12:20 -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 1uJvyO-0003dn-1T for qemu-devel@nongnu.org; Tue, 27 May 2025 11:12:14 -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 1uJvyG-0007Un-Ls for qemu-devel@nongnu.org; Tue, 27 May 2025 11:12:10 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1748358722; 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=/ga+4ynvGmz3YUt7Tv4iZYJWJsba7PhYx5/bGHZ310Q=; b=ENsuFYwmYAg+dZS0Ag5OyMrKDL6+o4SmuIoRHZA8FLHVmej0JxzDKCJxbMD0LcWl3t9LIa 1A4kj2GM/ITM4J/8Bqv3EV6XdH3/wh6v/7jMZK23wvUfYVpl1VTeSRxQBlRzrC+OY2rhE8 /QHVGhfdzuzs1eZhbW+BtXjDuA2vyp4= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-28-85RdAwdpOPqAoWd4fZFlzg-1; Tue, 27 May 2025 11:11:59 -0400 X-MC-Unique: 85RdAwdpOPqAoWd4fZFlzg-1 X-Mimecast-MFC-AGG-ID: 85RdAwdpOPqAoWd4fZFlzg_1748358717 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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 mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 332281800261; Tue, 27 May 2025 15:11:56 +0000 (UTC) Received: from redhat.com (unknown [10.44.34.20]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 783B518004A7; Tue, 27 May 2025 15:11:50 +0000 (UTC) Date: Tue, 27 May 2025 17:11:47 +0200 From: Kevin Wolf To: Fiona Ebner Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, den@virtuozzo.com, andrey.drobyshev@virtuozzo.com, hreitz@redhat.com, stefanha@redhat.com, eblake@redhat.com, jsnow@redhat.com, vsementsov@yandex-team.ru, xiechanglong.d@gmail.com, wencongyang2@huawei.com, berto@igalia.com, fam@euphon.net, ari@tuxera.com Subject: Re: [PATCH v3 08/24] block: move drain outside of bdrv_change_aio_context() and mark GRAPH_RDLOCK Message-ID: References: <20250526132140.1641377-1-f.ebner@proxmox.com> <20250526132140.1641377-9-f.ebner@proxmox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250526132140.1641377-9-f.ebner@proxmox.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 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: -49 X-Spam_score: -5.0 X-Spam_bar: ----- X-Spam_report: (-5.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.907, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=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 26.05.2025 um 15:21 hat Fiona Ebner geschrieben: > This is in preparation to mark bdrv_drained_begin() as GRAPH_UNLOCKED. > > Note that even if bdrv_drained_begin() were already marked as > GRAPH_UNLOCKED, TSA would not complain about the instance in > bdrv_change_aio_context() before this change, because it is preceded > by a bdrv_graph_rdunlock_main_loop() call. It is not correct to > release the lock here, and in case the caller holds a write lock, it > wouldn't actually release the lock. > > In combination with block-stream, there is a deadlock that can happen > because of this [0]. In particular, it can happen that > main thread IO thread > 1. acquires write lock > in blk_co_do_preadv_part(): > 2. have non-zero blk->in_flight > 3. try to acquire read lock > 4. begin drain > > Steps 3 and 4 might be switched. Draining will poll and get stuck, > because it will see the non-zero in_flight counter. But the IO thread > will not make any progress either, because it cannot acquire the read > lock. > > After this change, all paths to bdrv_change_aio_context() drain: > bdrv_change_aio_context() is called by: > 1. bdrv_child_cb_change_aio_ctx() which is only called via the > change_aio_ctx() callback, see below. > 2. bdrv_child_change_aio_context(), see below. > 3. bdrv_try_change_aio_context(), where a drained section is > introduced. > > The change_aio_ctx() callback is called by: > 1. bdrv_attach_child_common_abort(), where a drained section is > introduced. > 2. bdrv_attach_child_common(), where a drained section is introduced. > 3. bdrv_parent_change_aio_context(), see below. > > bdrv_child_change_aio_context() is called by: > 1. bdrv_change_aio_context(), i.e. recursive, so being in a drained > section is invariant. > 2. child_job_change_aio_ctx(), which is only called via the > change_aio_ctx() callback, see above. > > bdrv_parent_change_aio_context() is called by: > 1. bdrv_change_aio_context(), i.e. recursive, so being in a drained > section is invariant. > > This resolves all code paths. Note that bdrv_attach_child_common() > and bdrv_attach_child_common_abort() hold the graph write lock and > callers of bdrv_try_change_aio_context() might too, so they are not > actually allowed to drain either. This will be addressed in the > following commits. > > More granular draining is not trivially possible, because > bdrv_change_aio_context() can recursively call itself e.g. via > bdrv_child_change_aio_context(). > > [0]: https://lore.kernel.org/qemu-devel/73839c04-7616-407e-b057-80ca69e63f51@virtuozzo.com/ > > Reported-by: Andrey Drobyshev > Signed-off-by: Fiona Ebner Reviewed-by: Kevin Wolf