From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:53089) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hEu5a-0003h7-JL for qemu-devel@nongnu.org; Fri, 12 Apr 2019 07:11:23 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hEu5Y-0004Ua-B9 for qemu-devel@nongnu.org; Fri, 12 Apr 2019 07:11:21 -0400 Date: Fri, 12 Apr 2019 13:11:13 +0200 From: Kevin Wolf Message-ID: <20190412111113.GE4522@linux.fritz.box> References: <20190225152053.15976-1-kwolf@redhat.com> <20190225152053.15976-17-kwolf@redhat.com> <4f9792a1-4702-62d0-cad4-4da1169baa3b@virtuozzo.com> <20190411141548.GE5694@linux.fritz.box> <1391b876-74ed-21f0-c41f-f2fb22d2eae7@virtuozzo.com> <20190411164803.GF5694@linux.fritz.box> <2558f6fa-03d5-9483-07ff-7ec0a4d37835@virtuozzo.com> <0b088cc0-6ea9-297d-6989-2d61e1c25075@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0b088cc0-6ea9-297d-6989-2d61e1c25075@virtuozzo.com> Subject: Re: [Qemu-devel] [PULL 16/71] nbd: Increase bs->in_flight during AioContext switch List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vladimir Sementsov-Ogievskiy Cc: "qemu-block@nongnu.org" , "qemu-devel@nongnu.org" Am 11.04.2019 um 19:20 hat Vladimir Sementsov-Ogievskiy geschrieben: > 11.04.2019 20:13, Vladimir Sementsov-Ogievskiy wrote: > > 11.04.2019 19:48, Kevin Wolf wrote: > >> Am 11.04.2019 um 16:48 hat Vladimir Sementsov-Ogievskiy geschrieben: > >>> 11.04.2019 17:15, Kevin Wolf wrote: > >>>> Am 11.04.2019 um 15:40 hat Vladimir Sementsov-Ogievskiy geschrieben: > >>>>> 25.02.2019 18:19, Kevin Wolf wrote: > >>>>>> bdrv_drain() must not leave connection_co scheduled, so bs->in_flight > >>>>>> needs to be increased while the coroutine is waiting to be scheduled > >>>>>> in the new AioContext after nbd_client_attach_aio_context(). > >>>>> > >>>>> Hi! > >>>>> > >>>>> I have some questions, could you explain, please? > >>>>> > >>>>> "bdrv_drain() must not leave connection_co scheduled" - it's because we want to be > >>>>> sure that connection_co yielded from nbd_read_eof, yes? > >>>>> > >>>>> But it is guaranteed by aio_wait_bh_oneshot.. Why do we need additioinally inc/dec > >>>>> bs->in_flight ? > >>>> > >>>> Without incrementing bs->in_flight, nothing would guarantee that > >>>> aio_poll() is called and the BH is actually executed before bdrv_drain() > >>>> returns. > >>> > >>> Don't follow.. Don't we want exactly this, we want BH to be executed while node is still > >>> drained, as you write in comment? > >> > >> Yes, exactly. But if bs->in_flight == 0, the AIO_WAIT_WHILE() condition > >> in the drain code could become false, so aio_poll() would not be called > >> again and drain would return even if the BH is still pending. > >> > > > > Ah, oops, sorry my English, I read it like "nothing would prevent". Understand now, thanks. > > Or not again.. We will not return to drain code, as we will loop in aio_wait_bh_oneshot, > which will not return until BH handled Oh, hm, good point, it's not just aio_bh_schedule_oneshot(), but the waiting version. I think I added the aio_wait_bh_oneshot() only later to address a review comment, so maybe increasing bs->in_flight isn't needed any more now. But it can't hurt at least... Kevin 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=-2.4 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 64759C10F0E for ; Fri, 12 Apr 2019 11:12:18 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 3580920675 for ; Fri, 12 Apr 2019 11:12:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3580920675 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 ([127.0.0.1]:34409 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hEu6R-00044X-JP for qemu-devel@archiver.kernel.org; Fri, 12 Apr 2019 07:12:15 -0400 Received: from eggs.gnu.org ([209.51.188.92]:53089) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hEu5a-0003h7-JL for qemu-devel@nongnu.org; Fri, 12 Apr 2019 07:11:23 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hEu5Y-0004Ua-B9 for qemu-devel@nongnu.org; Fri, 12 Apr 2019 07:11:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38184) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hEu5V-0004TI-0V; Fri, 12 Apr 2019 07:11:17 -0400 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 mx1.redhat.com (Postfix) with ESMTPS id 1B1C13087954; Fri, 12 Apr 2019 11:11:16 +0000 (UTC) Received: from linux.fritz.box (unknown [10.36.118.26]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EBA1E5C207; Fri, 12 Apr 2019 11:11:14 +0000 (UTC) Date: Fri, 12 Apr 2019 13:11:13 +0200 From: Kevin Wolf To: Vladimir Sementsov-Ogievskiy Message-ID: <20190412111113.GE4522@linux.fritz.box> References: <20190225152053.15976-1-kwolf@redhat.com> <20190225152053.15976-17-kwolf@redhat.com> <4f9792a1-4702-62d0-cad4-4da1169baa3b@virtuozzo.com> <20190411141548.GE5694@linux.fritz.box> <1391b876-74ed-21f0-c41f-f2fb22d2eae7@virtuozzo.com> <20190411164803.GF5694@linux.fritz.box> <2558f6fa-03d5-9483-07ff-7ec0a4d37835@virtuozzo.com> <0b088cc0-6ea9-297d-6989-2d61e1c25075@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline In-Reply-To: <0b088cc0-6ea9-297d-6989-2d61e1c25075@virtuozzo.com> User-Agent: Mutt/1.11.3 (2019-02-01) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Fri, 12 Apr 2019 11:11:16 +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] [PULL 16/71] nbd: Increase bs->in_flight during AioContext switch X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "qemu-devel@nongnu.org" , "qemu-block@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Message-ID: <20190412111113.18Yro9Bo2y1RNljHxFXosvJrxku2ryyLzxdogUKfgtg@z> Am 11.04.2019 um 19:20 hat Vladimir Sementsov-Ogievskiy geschrieben: > 11.04.2019 20:13, Vladimir Sementsov-Ogievskiy wrote: > > 11.04.2019 19:48, Kevin Wolf wrote: > >> Am 11.04.2019 um 16:48 hat Vladimir Sementsov-Ogievskiy geschrieben: > >>> 11.04.2019 17:15, Kevin Wolf wrote: > >>>> Am 11.04.2019 um 15:40 hat Vladimir Sementsov-Ogievskiy geschrieben: > >>>>> 25.02.2019 18:19, Kevin Wolf wrote: > >>>>>> bdrv_drain() must not leave connection_co scheduled, so bs->in_flight > >>>>>> needs to be increased while the coroutine is waiting to be scheduled > >>>>>> in the new AioContext after nbd_client_attach_aio_context(). > >>>>> > >>>>> Hi! > >>>>> > >>>>> I have some questions, could you explain, please? > >>>>> > >>>>> "bdrv_drain() must not leave connection_co scheduled" - it's because we want to be > >>>>> sure that connection_co yielded from nbd_read_eof, yes? > >>>>> > >>>>> But it is guaranteed by aio_wait_bh_oneshot.. Why do we need additioinally inc/dec > >>>>> bs->in_flight ? > >>>> > >>>> Without incrementing bs->in_flight, nothing would guarantee that > >>>> aio_poll() is called and the BH is actually executed before bdrv_drain() > >>>> returns. > >>> > >>> Don't follow.. Don't we want exactly this, we want BH to be executed while node is still > >>> drained, as you write in comment? > >> > >> Yes, exactly. But if bs->in_flight == 0, the AIO_WAIT_WHILE() condition > >> in the drain code could become false, so aio_poll() would not be called > >> again and drain would return even if the BH is still pending. > >> > > > > Ah, oops, sorry my English, I read it like "nothing would prevent". Understand now, thanks. > > Or not again.. We will not return to drain code, as we will loop in aio_wait_bh_oneshot, > which will not return until BH handled Oh, hm, good point, it's not just aio_bh_schedule_oneshot(), but the waiting version. I think I added the aio_wait_bh_oneshot() only later to address a review comment, so maybe increasing bs->in_flight isn't needed any more now. But it can't hurt at least... Kevin