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=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 CFB06C433E0 for ; Mon, 18 Jan 2021 14:19:43 +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 3B4F622227 for ; Mon, 18 Jan 2021 14:19:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3B4F622227 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]:50758 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l1VNd-0008Dl-Rj for qemu-devel@archiver.kernel.org; Mon, 18 Jan 2021 09:19:41 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:50222) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l1VMW-0007mD-1h for qemu-devel@nongnu.org; Mon, 18 Jan 2021 09:18:32 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:37179) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l1VMT-00084w-4R for qemu-devel@nongnu.org; Mon, 18 Jan 2021 09:18:31 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1610979506; 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=ohbPKIRGpZmtdD4Y/d3s7MNOjiVVN03aBC04xwKSra8=; b=C3KaYopDnfQGFgushU6npWNDA0MZx2eNtxRxTzlr7oTdK0HA8HiR72Qh1iK8QhaDnkRSnS uqDETc0FhkZsI3F2HWmGtZ0RF2ApQsmUFk4tY71yInDVg+Bxws96lGP7in40Jx1WcSfpZu xx2tdUfUptkP5eBE8gbquDbYSBxYIfc= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-98-1OtfKgYnPauYPxQqF8b9_Q-1; Mon, 18 Jan 2021 09:18:25 -0500 X-MC-Unique: 1OtfKgYnPauYPxQqF8b9_Q-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C9FF81005513; Mon, 18 Jan 2021 14:18:23 +0000 (UTC) Received: from merkur.fritz.box (ovpn-115-75.ams2.redhat.com [10.36.115.75]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 470DB5D9CD; Mon, 18 Jan 2021 14:18:22 +0000 (UTC) Date: Mon, 18 Jan 2021 15:18:20 +0100 From: Kevin Wolf To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH v2 03/36] block: bdrv_append(): don't consume reference Message-ID: <20210118141820.GD11555@merkur.fritz.box> References: <20201127144522.29991-1-vsementsov@virtuozzo.com> <20201127144522.29991-4-vsementsov@virtuozzo.com> MIME-Version: 1.0 In-Reply-To: <20201127144522.29991-4-vsementsov@virtuozzo.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 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=63.128.21.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.175, 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 autolearn=unavailable autolearn_force=no X-Spam_action: no action 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: qemu-block@nongnu.org, armbru@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, den@openvz.org, jsnow@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 27.11.2020 um 15:44 hat Vladimir Sementsov-Ogievskiy geschrieben: > We have too much comments for this feature. It seems better just don't > do it. Most of real users (tests don't count) have to create additional > reference. > > Drop also comment in external_snapshot_prepare: > - bdrv_append doesn't "remove" old bs in common sense, it sounds > strange > - the fact that bdrv_append can fail is obvious from the context > - the fact that we must rollback all changes in transaction abort is > known (it's the direct role of abort) > > Signed-off-by: Vladimir Sementsov-Ogievskiy > --- > block.c | 19 +++---------------- > block/backup-top.c | 1 - > block/commit.c | 1 + > block/mirror.c | 3 --- > blockdev.c | 4 ---- > tests/test-bdrv-drain.c | 2 +- > tests/test-bdrv-graph-mod.c | 2 ++ > 7 files changed, 7 insertions(+), 25 deletions(-) > > diff --git a/block.c b/block.c > index 0dd28f0902..55efef3c9d 100644 > --- a/block.c > +++ b/block.c > @@ -3145,11 +3145,6 @@ static BlockDriverState *bdrv_append_temp_snapshot(BlockDriverState *bs, > goto out; > } > > - /* bdrv_append() consumes a strong reference to bs_snapshot > - * (i.e. it will call bdrv_unref() on it) even on error, so in > - * order to be able to return one, we have to increase > - * bs_snapshot's refcount here */ > - bdrv_ref(bs_snapshot); > bdrv_append(bs_snapshot, bs, &local_err); > if (local_err) { > error_propagate(errp, local_err); > @@ -4608,10 +4603,8 @@ void bdrv_replace_node(BlockDriverState *from, BlockDriverState *to, > * > * This function does not create any image files. > * > - * bdrv_append() takes ownership of a bs_new reference and unrefs it because > - * that's what the callers commonly need. bs_new will be referenced by the old > - * parents of bs_top after bdrv_append() returns. If the caller needs to keep a > - * reference of its own, it must call bdrv_ref(). > + * Recent update: bdrv_append does NOT eat bs_new reference for now. Drop this > + * comment several moths later. A comment like this is unusual. Do you think there is a high risk of somebody introducing a new bdrv_append() in parallel and that they would read this comment when rebasing their existing patches? If we do keep the comment: s/for now/now/ (it has recently changed, we're not intending to change it later) and s/moths/months/. > */ > void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top, > Error **errp) > @@ -4621,20 +4614,14 @@ void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top, > bdrv_set_backing_hd(bs_new, bs_top, &local_err); > if (local_err) { > error_propagate(errp, local_err); > - goto out; > + return; > } > > bdrv_replace_node(bs_top, bs_new, &local_err); > if (local_err) { > error_propagate(errp, local_err); > bdrv_set_backing_hd(bs_new, NULL, &error_abort); > - goto out; Can we leave a return here just in case that new code will be added at the end of the function? > } > - > - /* bs_new is now referenced by its new parents, we don't need the > - * additional reference any more. */ > -out: > - bdrv_unref(bs_new); > } > > static void bdrv_delete(BlockDriverState *bs) > diff --git a/block/backup-top.c b/block/backup-top.c > index fe6883cc97..650ed6195c 100644 > --- a/block/backup-top.c > +++ b/block/backup-top.c > @@ -222,7 +222,6 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState *source, > > bdrv_drained_begin(source); > > - bdrv_ref(top); > bdrv_append(top, source, &local_err); > if (local_err) { > error_prepend(&local_err, "Cannot append backup-top filter: "); > diff --git a/block/commit.c b/block/commit.c > index 71db7ba747..61924bcf66 100644 > --- a/block/commit.c > +++ b/block/commit.c > @@ -313,6 +313,7 @@ void commit_start(const char *job_id, BlockDriverState *bs, > commit_top_bs->total_sectors = top->total_sectors; > > bdrv_append(commit_top_bs, top, &local_err); > + bdrv_unref(commit_top_bs); /* referenced by new parents or failed */ > if (local_err) { > commit_top_bs = NULL; > error_propagate(errp, local_err); > diff --git a/block/mirror.c b/block/mirror.c > index 8e1ad6eceb..13f7ecc998 100644 > --- a/block/mirror.c > +++ b/block/mirror.c > @@ -1605,9 +1605,6 @@ static BlockJob *mirror_start_job( > bs_opaque = g_new0(MirrorBDSOpaque, 1); > mirror_top_bs->opaque = bs_opaque; > > - /* bdrv_append takes ownership of the mirror_top_bs reference, need to keep > - * it alive until block_job_create() succeeds even if bs has no parent. */ > - bdrv_ref(mirror_top_bs); > bdrv_drained_begin(bs); > bdrv_append(mirror_top_bs, bs, &local_err); > bdrv_drained_end(bs); > diff --git a/blockdev.c b/blockdev.c > index b5f11c524b..96c96f8ba6 100644 > --- a/blockdev.c > +++ b/blockdev.c > @@ -1587,10 +1587,6 @@ static void external_snapshot_prepare(BlkActionState *common, > goto out; > } > > - /* This removes our old bs and adds the new bs. This is an operation that > - * can fail, so we need to do it in .prepare; undoing it for abort is > - * always possible. */ This comment is still relevant, it's unrelated to the bdrv_ref(). > - bdrv_ref(state->new_bs); > bdrv_append(state->new_bs, state->old_bs, &local_err); > if (local_err) { > error_propagate(errp, local_err); Kevin