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 CF55DCCFA18 for ; Tue, 11 Nov 2025 14:20:23 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vIpDp-0007Sk-Ti; Tue, 11 Nov 2025 09:19:49 -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 1vIpCh-0006N0-1c for qemu-devel@nongnu.org; Tue, 11 Nov 2025 09:18:40 -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 1vIpCe-0002qg-5g for qemu-devel@nongnu.org; Tue, 11 Nov 2025 09:18:37 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1762870715; h=from:from:reply-to: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=CL+Cmq6fFUqzDhIKDYLGhHHPDC8Ib7bC/iDnGoWjnhg=; b=TpkDLkN1xuCDUhAk7Tl2pXzdZuBiTlaXhb7xJ8AzrgR6o59KnFL4kLQUWqbfIMjU90zEdY uO2GoO31MFEy6VPNCHZ+1VaNatrHeUl8QDomrKDWIS1qfo94jRV2DsYSD+vTOWuRIVZHOp gSZD7Y2u5XFKqHIGtVsRJyeMY3L//Jg= Received: from mx-prod-mc-08.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-619-fs5OrdmVMbm9ySYi89qi1g-1; Tue, 11 Nov 2025 09:18:33 -0500 X-MC-Unique: fs5OrdmVMbm9ySYi89qi1g-1 X-Mimecast-MFC-AGG-ID: fs5OrdmVMbm9ySYi89qi1g_1762870713 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 8C2871800473; Tue, 11 Nov 2025 14:18:32 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.84]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0E22630044E0; Tue, 11 Nov 2025 14:18:30 +0000 (UTC) Date: Tue, 11 Nov 2025 14:18:27 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Eric Blake Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, kwolf@redhat.com Subject: Re: [PATCH v2 10/12] qio: Add QIONetListener API for using AioContext Message-ID: References: <20251108230525.3169174-14-eblake@redhat.com> <20251108230525.3169174-24-eblake@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20251108230525.3169174-24-eblake@redhat.com> User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@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, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Sat, Nov 08, 2025 at 04:59:31PM -0600, Eric Blake wrote: > The user "John Doe" reported a deadlock when attempting to use > qemu-storage-daemon to serve both a base file over NBD, and a qcow2 > file with that NBD export as its backing file, from the same process, > even though it worked just fine when there were two q-s-d processes. > The bulk of the NBD server code properly uses coroutines to make > progress in an event-driven manner, but the code for spawning a new > coroutine at the point when listen(2) detects a new client was > hard-coded to use the global GMainContext; in other words, the > callback that triggers nbd_client_new to let the server start the > negotiation sequence with the client requires the main loop to be > making progress. However, the code for bdrv_open of a qcow2 image > with an NBD backing file uses an AIO_WAIT_WHILE nested event loop to > ensure that the entire qcow2 backing chain is either fully loaded or > rejected, without any side effects from the main loop causing unwanted > changes to the disk being loaded (in short, an AioContext represents > the set of actions that are known to be safe while handling block > layer I/O, while excluding any other pending actions in the global > main loop with potentially larger risk of unwanted side effects). > > This creates a classic case of deadlock: the server can't progress to > the point of accept(2)ing the client to write to the NBD socket > because the main loop is being starved until the AIO_WAIT_WHILE > completes the bdrv_open, but the AIO_WAIT_WHILE can't progress because > it is blocked on the client coroutine stuck in a read() of the > expected magic number from the server side of the socket. > > This patch adds a new API to allow clients to opt in to listening via > an AioContext rather than a GMainContext. This will allow NBD to fix > the deadlock by performing all actions during bdrv_open in the main > loop AioContext. > > An upcoming patch will then add a unit test (kept separate to make it > easier to rearrange the series to demonstrate the deadlock without > this patch). > > Fixes: https://gitlab.com/qemu-project/qemu/-/issues/3169 > Signed-off-by: Eric Blake > > --- > v2: Retitle and add new API rather than changing semantics of > existing qio_net_listener_set_client_func; use qio accessor rather > than direct access to the sioc fd and a lower-level aio call > --- > include/io/net-listener.h | 16 ++++++++++++++++ > io/net-listener.c | 36 +++++++++++++++++++++++++++++++++--- > 2 files changed, 49 insertions(+), 3 deletions(-) Reviewed-by: Daniel P. Berrangé With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|