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 X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 611F1C49ED7 for ; Fri, 13 Sep 2019 09:47:05 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 315B120717 for ; Fri, 13 Sep 2019 09:47:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 315B120717 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:41842 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i8iAS-0005Fr-7D for qemu-devel@archiver.kernel.org; Fri, 13 Sep 2019 05:47:04 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:56918) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1i8i9F-0004X1-9g for qemu-devel@nongnu.org; Fri, 13 Sep 2019 05:45:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1i8i9D-000344-Tw for qemu-devel@nongnu.org; Fri, 13 Sep 2019 05:45:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49602) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1i8i9A-00031K-NO; Fri, 13 Sep 2019 05:45:44 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id B55AC8980EE; Fri, 13 Sep 2019 09:45:43 +0000 (UTC) Received: from dhcp-200-226.str.redhat.com (dhcp-200-226.str.redhat.com [10.33.200.226]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 22BB36012A; Fri, 13 Sep 2019 09:45:34 +0000 (UTC) Date: Fri, 13 Sep 2019 11:45:33 +0200 From: Kevin Wolf To: Sergio Lopez Message-ID: <20190913094533.GB8312@dhcp-200-226.str.redhat.com> References: <20190912181924.48539-1-slp@redhat.com> <20190912155023-mutt-send-email-mst@kernel.org> <20190913090457.GA8312@dhcp-200-226.str.redhat.com> <87woecwnmv.fsf@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="pWyiEgJYm5f9v55/" Content-Disposition: inline In-Reply-To: <87woecwnmv.fsf@redhat.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.67]); Fri, 13 Sep 2019 09:45:43 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [RFC PATCH] virtio-blk: schedule virtio_notify_config to run on main context X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: mreitz@redhat.com, stefanha@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, "Michael S. Tsirkin" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --pWyiEgJYm5f9v55/ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 13.09.2019 um 11:28 hat Sergio Lopez geschrieben: >=20 > Kevin Wolf writes: >=20 > > Am 12.09.2019 um 21:51 hat Michael S. Tsirkin geschrieben: > >> On Thu, Sep 12, 2019 at 08:19:25PM +0200, Sergio Lopez wrote: > >> > Another AioContext-related issue, and this is a tricky one. > >> >=20 > >> > Executing a QMP block_resize request for a virtio-blk device running > >> > on an iothread may cause a deadlock involving the following mutexes: > >> >=20 > >> > - main thead > >> > * Has acquired: qemu_mutex_global. > >> > * Is trying the acquire: iothread AioContext lock via > >> > AIO_WAIT_WHILE (after aio_poll). > >> >=20 > >> > - iothread > >> > * Has acquired: AioContext lock. > >> > * Is trying to acquire: qemu_mutex_global (via > >> > virtio_notify_config->prepare_mmio_access). > >>=20 > >> Hmm is this really the only case iothread takes qemu mutex? > >> If any such access can deadlock, don't we need a generic > >> solution? Maybe main thread can drop qemu mutex > >> before taking io thread AioContext lock? > > > > The rule is that iothreads must not take the qemu mutex. If they do > > (like in this case), it's a bug. > > > > Maybe we could actually assert this in qemu_mutex_lock_iothread()? > > > >> > With this change, virtio_blk_resize checks if it's being called from= a > >> > coroutine context running on a non-main thread, and if that's the > >> > case, creates a new coroutine and schedules it to be run on the main > >> > thread. > >> >=20 > >> > This works, but means the actual operation is done > >> > asynchronously, perhaps opening a window in which a "device_del" > >> > operation may fit and remove the VirtIODevice before > >> > virtio_notify_config() is executed. > >> >=20 > >> > I *think* it shouldn't be possible, as BHs will be processed before > >> > any new QMP/monitor command, but I'm open to a different approach. > >> >=20 > >> > RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=3D1744955 > >> > Signed-off-by: Sergio Lopez > >> > --- > >> > hw/block/virtio-blk.c | 25 ++++++++++++++++++++++++- > >> > 1 file changed, 24 insertions(+), 1 deletion(-) > >> >=20 > >> > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c > >> > index 18851601cb..c763d071f6 100644 > >> > --- a/hw/block/virtio-blk.c > >> > +++ b/hw/block/virtio-blk.c > >> > @@ -16,6 +16,7 @@ > >> > #include "qemu/iov.h" > >> > #include "qemu/module.h" > >> > #include "qemu/error-report.h" > >> > +#include "qemu/main-loop.h" > >> > #include "trace.h" > >> > #include "hw/block/block.h" > >> > #include "hw/qdev-properties.h" > >> > @@ -1086,11 +1087,33 @@ static int virtio_blk_load_device(VirtIODevi= ce *vdev, QEMUFile *f, > >> > return 0; > >> > } > >> > =20 > >> > +static void coroutine_fn virtio_resize_co_entry(void *opaque) > >> > +{ > >> > + VirtIODevice *vdev =3D opaque; > >> > + > >> > + assert(qemu_get_current_aio_context() =3D=3D qemu_get_aio_conte= xt()); > >> > + virtio_notify_config(vdev); > >> > + aio_wait_kick(); > >> > +} > >> > + > >> > static void virtio_blk_resize(void *opaque) > >> > { > >> > VirtIODevice *vdev =3D VIRTIO_DEVICE(opaque); > >> > + Coroutine *co; > >> > =20 > >> > - virtio_notify_config(vdev); > >> > + if (qemu_in_coroutine() && > >> > + qemu_get_current_aio_context() !=3D qemu_get_aio_context())= { > >> > + /* > >> > + * virtio_notify_config() needs to acquire the global mutex, > >> > + * so calling it from a coroutine running on a non-main con= text > >> > + * may cause a deadlock. Instead, create a new coroutine and > >> > + * schedule it to be run on the main thread. > >> > + */ > >> > + co =3D qemu_coroutine_create(virtio_resize_co_entry, vdev); > >> > + aio_co_schedule(qemu_get_aio_context(), co); > >> > + } else { > >> > + virtio_notify_config(vdev); > >> > + } > >> > } > > > > Wouldn't a simple BH suffice (aio_bh_schedule_oneshot)? I don't see why > > you need a coroutine when you never yield. >=20 > You're right, that's actually simpler, haven't thought of it. >=20 > Do you see any drawbacks or should I send a non-RFC fixed version of > this patch? Sending a fixed non-RFC version sounds good to me. 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