From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42666) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZmY8s-000090-Pu for qemu-devel@nongnu.org; Wed, 14 Oct 2015 22:19:43 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZmY8o-000576-RH for qemu-devel@nongnu.org; Wed, 14 Oct 2015 22:19:42 -0400 References: <1443161858-20533-1-git-send-email-wency@cn.fujitsu.com> <1443161858-20533-9-git-send-email-wency@cn.fujitsu.com> <20151012162714.GC4053@stefanha-thinkpad.redhat.com> <561CCA01.7000001@cn.fujitsu.com> <20151014142751.GD16162@stefanha-thinkpad.redhat.com> From: Wen Congyang Message-ID: <561F0D25.9000409@cn.fujitsu.com> Date: Thu, 15 Oct 2015 10:19:17 +0800 MIME-Version: 1.0 In-Reply-To: <20151014142751.GD16162@stefanha-thinkpad.redhat.com> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , Fam Zheng , zhanghailiang , qemu block , Stefan Hajnoczi , Jeff Cody , Jiang Yunhong , Dong Eddie , qemu devel , "Michael R. Hines" , Max Reitz , Gonglei , Paolo Bonzini , Yang Hongyang , "Dr. David Alan Gilbert" On 10/14/2015 10:27 PM, Stefan Hajnoczi wrote: > On Tue, Oct 13, 2015 at 05:08:17PM +0800, Wen Congyang wrote: >> On 10/13/2015 12:27 AM, Stefan Hajnoczi wrote: >>> On Fri, Sep 25, 2015 at 02:17:36PM +0800, Wen Congyang wrote: >>>> + /* start backup job now */ >>>> + bdrv_op_unblock(s->hidden_disk, BLOCK_OP_TYPE_BACKUP_TARGET, >>>> + s->active_disk->backing_blocker); >>>> + bdrv_op_unblock(s->secondary_disk, BLOCK_OP_TYPE_BACKUP_SOURCE, >>>> + s->hidden_disk->backing_blocker); >>> >>> Why is it safe to unblock these operations? >>> >>> Why do they have to be blocked for non-replication users? >> >> hidden_disk and secondary disk are opened as backing file, so it is blocked for >> non-replication users. >> What can I do if I don't unblock it and want to do backup? > > CCing Jeff Cody, block jobs maintainer > > You need to explain why it is safe remove this protection. We can't > merge code that may be unsafe. > > I think we can investigate further by asking: when does QEMU code assume > the backing file is read-only? The backing file is opened in read-only mode. I want to reopen it in read-write mode here in the next version(So the patch 1 will be dropped) > > I haven't checked but these cases come to mind: > > Operations that move data between BDS in the backing chain (e.g. commit > and stream block jobs) will lose or overwrite data if the backing file > is being written to by another coroutine. > > We need to prevent users from running these operations at the same time. Yes, but qemu doesn't provide such API. > > Also, accessing bs->backing_blocker is a layering violation. No one > outside block.c:bdrv_set_backing_hd() is supposed to access this field. I agree with it. Thanks Wen Congyang > > Let's figure out the safety concerns first and then the > bs->backing_blocker access will probably be eliminated as part of the > solution. > > Stefan > . >