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 EFC4DC433EF for ; Fri, 11 Feb 2022 11:56:51 +0000 (UTC) Received: from localhost ([::1]:53800 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nIUXi-0002f2-Mi for qemu-devel@archiver.kernel.org; Fri, 11 Feb 2022 06:56:50 -0500 Received: from eggs.gnu.org ([209.51.188.92]:35138) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nIUVf-00017a-Qc for qemu-devel@nongnu.org; Fri, 11 Feb 2022 06:54:43 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:31534) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nIUVc-0007g2-Io for qemu-devel@nongnu.org; Fri, 11 Feb 2022 06:54:42 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1644580478; 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=w72ONOiYvw75ZOHe2ORIlweVqO3P1Xf/1XXkGdpxqiE=; b=SSDnD4Xu0FCutO2IR01Tj4llybHbvvaJYFVyV9FjL5GYFKemsRpehqFa5zSABc/EGZqkAc 1gY3SeGB1UYFZoQaAH0TTn6wK3lvzLOXJPoPbiWimH2Ijq5ma68J63KB6wcANDNzOrQx1h X4rOE4yGOe4W4hwtSN4n0oxpvF0dpIw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-202-EKOnlJk_OAS_WN4-3a-33w-1; Fri, 11 Feb 2022 06:54:35 -0500 X-MC-Unique: EKOnlJk_OAS_WN4-3a-33w-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 6514E192D787; Fri, 11 Feb 2022 11:54:34 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.97]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1A09E7C0F9; Fri, 11 Feb 2022 11:54:31 +0000 (UTC) Date: Fri, 11 Feb 2022 12:54:30 +0100 From: Kevin Wolf To: Emanuele Giuseppe Esposito Subject: Re: [PATCH 1/6] block/io.c: fix bdrv_child_cb_drained_begin invocations from a coroutine Message-ID: References: <20220208153655.1251658-1-eesposit@redhat.com> <20220208153655.1251658-2-eesposit@redhat.com> MIME-Version: 1.0 In-Reply-To: <20220208153655.1251658-2-eesposit@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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: , Cc: Fam Zheng , Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org, qemu-devel@nongnu.org, Hanna Reitz , Stefan Hajnoczi , Paolo Bonzini , John Snow Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 08.02.2022 um 16:36 hat Emanuele Giuseppe Esposito geschrieben: > Using bdrv_do_drained_begin_quiesce() in bdrv_child_cb_drained_begin() > is not a good idea: the callback might be called when running > a drain in a coroutine, and bdrv_drained_begin_poll() does not > handle that case, resulting in assertion failure. I remembered that we talked about this only recently on IRC, but it didn't make any sense to me again when I read this commit message. So I think we need --verbose. The .drained_begin callback was always meant to run outside of coroutine context, so the unexpected part isn't that it calls a function that can't run in coroutine context, but that it is already called itself in coroutine context. The problematic path is bdrv_replace_child_noperm() which then calls bdrv_parent_drained_begin_single(poll=true). Polling in coroutine context is dangerous, it can cause deadlocks because the caller of the coroutine can't make progress. So I believe this call is already wrong in coroutine context. Now I don't know the call path up to bdrv_replace_child_noperm(), but as far as I remember, that was another function that was originally never run in coroutine context. Maybe we have good reason to change this, I can't point to anything that would be inherently wrong with it, but I would still be curious in which context it does run in a coroutine now. Anyway, whatever the specific place is, I believe we must drop out of coroutine context _before_ calling bdrv_parent_drained_begin_single(), not only in callbacks called by it. > Instead, bdrv_do_drained_begin with no recursion and poll > will accomplish the same thing (invoking bdrv_do_drained_begin_quiesce) > but will firstly check if we are already in a coroutine, and exit > from that via bdrv_co_yield_to_drain(). > > Signed-off-by: Emanuele Giuseppe Esposito Kevin