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 BC9A1C433F5 for ; Tue, 5 Apr 2022 09:44:18 +0000 (UTC) Received: from localhost ([::1]:48412 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nbfjV-0004yf-IU for qemu-devel@archiver.kernel.org; Tue, 05 Apr 2022 05:44:17 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:46948) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nbffP-0003CK-4R for qemu-devel@nongnu.org; Tue, 05 Apr 2022 05:40:03 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:34124) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nbffM-0007sD-Eu for qemu-devel@nongnu.org; Tue, 05 Apr 2022 05:40:01 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1649151599; 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=nrYa9sUqSln5BlV4oX1sXA10ZfPvKOOyzeD33RJTsgU=; b=OAWRw8ixkSrDBPyp82Ym7c08CwvJdNa07er/YnxD0PwbJ1rpfEv1LwXvlo/vR5z+10si4p +khXRfJ+LCMMHUzPj+/p30n6bdIpUqaeGz7mVX5wKzqrWF3w4yrt2Dx4zfS60GhwpeK1BY UcOkeX843FiBnUmMhHffZwvC6LK5ZWo= 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-203-7wL0Uv2UNTGrKGnkDybJVw-1; Tue, 05 Apr 2022 05:39:56 -0400 X-MC-Unique: 7wL0Uv2UNTGrKGnkDybJVw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id DF571811E75; Tue, 5 Apr 2022 09:39:55 +0000 (UTC) Received: from localhost (unknown [10.39.193.238]) by smtp.corp.redhat.com (Postfix) with ESMTP id E283AFFE1; Tue, 5 Apr 2022 09:39:52 +0000 (UTC) Date: Tue, 5 Apr 2022 10:39:51 +0100 From: Stefan Hajnoczi To: Paolo Bonzini Subject: Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept Message-ID: References: <88f2798b-9327-e54f-5792-e37404b94ef7@redhat.com> <8ae70388-ff46-6ec1-7f84-14d41ca9a6dd@redhat.com> <311c2e0a-fb2c-241c-cbd1-1162f7e74e18@redhat.com> <9d3c36f0-0834-ec9c-8473-d052d64a61dd@redhat.com> <69b2ce82-4826-71ed-9c32-d323df69b7c4@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="ZIO8P9w2A6tpIu7x" Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 Received-SPF: pass client-ip=170.10.129.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, 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: , Cc: Emanuele Giuseppe Esposito , Fam Zheng , Vladimir Sementsov-Ogievskiy , "open list:Block layer core" , qemu-devel , Hanna Reitz , Kevin Wolf , John Snow Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --ZIO8P9w2A6tpIu7x Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Apr 04, 2022 at 11:41:04AM +0200, Paolo Bonzini wrote: > On Mon, Apr 4, 2022 at 11:25 AM Stefan Hajnoczi wro= te: > > - The new API still needs to be combined with bdrv_drained_begin/end() > > to ensure in-flight requests are done. > > >=20 > I don't think so, because in-flight requests would take the lock for > reading. The write side would not start until those in-flight requests > release the lock. Good point! > - It's not obvious to me whether the new API obsoletes is_external. I thi= nk > > it probably doesn't. > > >=20 > I agree that it doesn't. This new lock is only protecting ->parents and > ->children. bdrv_drained_begin()/end() remains necessary, for example, wh= en > you need to send a request during the drained section. An example is > block_resize. >=20 > In addition, bdrv_drained_begin()/end() ensures that the callback of > blk_aio_*() functions has been invoked (see commit 46aaf2a566, > "block-backend: Decrease in_flight only after callback", 2018-09-25). Th= is > new lock would not ensure that. >=20 > As an aside, instead of is_external, QEMU could remove/add the ioeventfd > handler in the blk->dev_ops->drained_begin and blk->dev_ops->drained_end > callbacks respectively. But that's just a code cleanup. Interesting idea. If is_external is a block layer-specific feature that nothing else in QEMU uses then I like the idea because it's cleaner and more obvious than is_external. Stefan --ZIO8P9w2A6tpIu7x Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmJMDmcACgkQnKSrs4Gr c8ilpwf/cSj8jx2yTFGcv/W19pDyhuM+yWFw5pzU9KPA86xJ8ltsex910HMYk3sn DaiwQLZuNTofbwr3sJ+319U5/FHuCk4it6g/tQ0R6sqWgFKns6T4Fa04l/CRIoF8 pOPrxTHlmUeLlglLePGVwIs5NQ4UiVqh8QFLZcvQk7qgDPB9E7FqPTaiLUr/9Lt6 xp22YolAlnwoWNgslwuB3G4PScD2pbfLwy0atfOTfbs8WKfrRg7SI1FK/O0fFGEX 9Ecc6/qcMq1hY7Sn+hyfXYtJ7r+Lek3F2lOaMYgyzhupt/RLNfS3rnOwZ+1/66Mn EDKo2pS7wiVwnL8ATRUJs3+ibEhY8A== =XqtJ -----END PGP SIGNATURE----- --ZIO8P9w2A6tpIu7x--