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 4EFFBC77B75 for ; Wed, 3 May 2023 13:43:45 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1puClK-0005VQ-C3; Wed, 03 May 2023 09:43:18 -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 1puClJ-0005Ux-0x for qemu-devel@nongnu.org; Wed, 03 May 2023 09:43:17 -0400 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 1puClH-00048c-Bb for qemu-devel@nongnu.org; Wed, 03 May 2023 09:43:16 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1683121394; 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=LnWGqGWWdJPHGGU1rFwcwMAX+ugZvyvG/DLBRKCKxQk=; b=U7UJHYbecJD9rXD7Fts4+fnfSHTfowFdOFjCtbByy/ojv1PgG9emOTzBb7Xq+Sn+iHpJn0 bu6L7o+x+uBT4qsMXtoFlbSodno4p7TevMxzgfxGhQ5C6cFWvZsdrFfgjO9HcHjv9RXcxZ nH9OvwPLLFNkhMdXUqn4JZNMZzjz0mw= 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-63-BMx0iVPUPgGDcLgQ7YyuzA-1; Wed, 03 May 2023 09:43:10 -0400 X-MC-Unique: BMx0iVPUPgGDcLgQ7YyuzA-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 92812852AF6; Wed, 3 May 2023 13:43:09 +0000 (UTC) Received: from redhat.com (dhcp-192-205.str.redhat.com [10.33.192.205]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 164622166B29; Wed, 3 May 2023 13:43:05 +0000 (UTC) Date: Wed, 3 May 2023 15:43:05 +0200 From: Kevin Wolf To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Juan Quintela , Julia Suvorova , xen-devel@lists.xenproject.org, eesposit@redhat.com, Richard Henderson , Fam Zheng , "Michael S. Tsirkin" , Coiby Xu , David Woodhouse , Marcel Apfelbaum , Peter Lieven , Paul Durrant , "Richard W.M. Jones" , qemu-block@nongnu.org, Stefano Garzarella , Anthony Perard , Stefan Weil , Xie Yongji , Paolo Bonzini , Aarushi Mehta , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Eduardo Habkost , Stefano Stabellini , Hanna Reitz , Ronnie Sahlberg Subject: Re: [PATCH v4 07/20] block/export: stop using is_external in vhost-user-blk server Message-ID: References: <20230425172716.1033562-1-stefanha@redhat.com> <20230425172716.1033562-8-stefanha@redhat.com> <20230502200645.GE535070@fedora> <20230503131125.GB757667@fedora> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="3jXylBdepGNQTKaY" Content-Disposition: inline In-Reply-To: <20230503131125.GB757667@fedora> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -22 X-Spam_score: -2.3 X-Spam_bar: -- X-Spam_report: (-2.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.161, 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, 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --3jXylBdepGNQTKaY Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 03.05.2023 um 15:11 hat Stefan Hajnoczi geschrieben: > On Wed, May 03, 2023 at 10:08:46AM +0200, Kevin Wolf wrote: > > Am 02.05.2023 um 22:06 hat Stefan Hajnoczi geschrieben: > > > On Tue, May 02, 2023 at 06:04:24PM +0200, Kevin Wolf wrote: > > > > Am 25.04.2023 um 19:27 hat Stefan Hajnoczi geschrieben: > > > > > vhost-user activity must be suspended during bdrv_drained_begin/e= nd(). > > > > > This prevents new requests from interfering with whatever is happ= ening > > > > > in the drained section. > > > > >=20 > > > > > Previously this was done using aio_set_fd_handler()'s is_external > > > > > argument. In a multi-queue block layer world the aio_disable_exte= rnal() > > > > > API cannot be used since multiple AioContext may be processing I/= O, not > > > > > just one. > > > > >=20 > > > > > Switch to BlockDevOps->drained_begin/end() callbacks. > > > > >=20 > > > > > Signed-off-by: Stefan Hajnoczi > > > > > --- > > > > > block/export/vhost-user-blk-server.c | 43 ++++++++++++++--------= ------ > > > > > util/vhost-user-server.c | 10 +++---- > > > > > 2 files changed, 26 insertions(+), 27 deletions(-) > > > > >=20 > > > > > diff --git a/block/export/vhost-user-blk-server.c b/block/export/= vhost-user-blk-server.c > > > > > index 092b86aae4..d20f69cd74 100644 > > > > > --- a/block/export/vhost-user-blk-server.c > > > > > +++ b/block/export/vhost-user-blk-server.c > > > > > @@ -208,22 +208,6 @@ static const VuDevIface vu_blk_iface =3D { > > > > > .process_msg =3D vu_blk_process_msg, > > > > > }; > > > > > =20 > > > > > -static void blk_aio_attached(AioContext *ctx, void *opaque) > > > > > -{ > > > > > - VuBlkExport *vexp =3D opaque; > > > > > - > > > > > - vexp->export.ctx =3D ctx; > > > > > - vhost_user_server_attach_aio_context(&vexp->vu_server, ctx); > > > > > -} > > > > > - > > > > > -static void blk_aio_detach(void *opaque) > > > > > -{ > > > > > - VuBlkExport *vexp =3D opaque; > > > > > - > > > > > - vhost_user_server_detach_aio_context(&vexp->vu_server); > > > > > - vexp->export.ctx =3D NULL; > > > > > -} > > > >=20 > > > > So for changing the AioContext, we now rely on the fact that the no= de to > > > > be changed is always drained, so the drain callbacks implicitly cov= er > > > > this case, too? > > >=20 > > > Yes. > >=20 > > Ok. This surprised me a bit at first, but I think it's fine. > >=20 > > We just need to remember it if we ever decide that once we have > > multiqueue, we can actually change the default AioContext without > > draining the node. But maybe at that point, we have to do more > > fundamental changes anyway. > >=20 > > > > > static void > > > > > vu_blk_initialize_config(BlockDriverState *bs, > > > > > struct virtio_blk_config *config, > > > > > @@ -272,6 +256,25 @@ static void vu_blk_exp_resize(void *opaque) > > > > > vu_config_change_msg(&vexp->vu_server.vu_dev); > > > > > } > > > > > =20 > > > > > +/* Called with vexp->export.ctx acquired */ > > > > > +static void vu_blk_drained_begin(void *opaque) > > > > > +{ > > > > > + VuBlkExport *vexp =3D opaque; > > > > > + > > > > > + vhost_user_server_detach_aio_context(&vexp->vu_server); > > > > > +} > > > >=20 > > > > Compared to the old code, we're losing the vexp->export.ctx =3D NUL= L. This > > > > is correct at this point because after drained_begin we still keep > > > > processing requests until we arrive at a quiescent state. > > > >=20 > > > > However, if we detach the AioContext because we're deleting the > > > > iothread, won't we end up with a dangling pointer in vexp->export.c= tx? > > > > Or can we be certain that nothing interesting happens before draine= d_end > > > > updates it with a new valid pointer again? > > >=20 > > > If you want I can add the detach() callback back again and set ctx to > > > NULL there? > >=20 > > I haven't thought enough about it to say if it's a problem. If you have > > and are confident that it's correct the way it is, I'm happy with it. > > > > But bringing the callback back is the minimal change compared to the old > > state. It's just unnecessary code if we don't actually need it. >=20 > The reasoning behind my patch is that detach() sets NULL today and we > would see crashes if ctx was accessed between detach() -> attach(). > Therefore, I'm assuming there are no ctx accesses in the code today and > removing the ctx =3D NULL assignment doesn't break anything. Sometimes ctx =3D NULL defaults to qemu_get_aio_context(), so in theory there could be cases where NULL works, but a dangling pointer wouldn't. > However, my approach is not very defensive. If the code is changed in > a way that accesses ctx when it's not supposed to, then a dangling > pointer will be accessed. >=20 > I think leaving the detach() callback there can be justified because it > will make it easier to detect bugs in the future. I'll add it back in > the next revision. Ok, sounds good me. 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