From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:51813) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VK9G0-0001vi-1X for qemu-devel@nongnu.org; Thu, 12 Sep 2013 11:56:41 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VK9Fu-0005Gr-Ja for qemu-devel@nongnu.org; Thu, 12 Sep 2013 11:56:35 -0400 Received: from mail-bk0-f42.google.com ([209.85.214.42]:52597) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VK9Fu-0005Gj-DM for qemu-devel@nongnu.org; Thu, 12 Sep 2013 11:56:30 -0400 Received: by mail-bk0-f42.google.com with SMTP id my10so4381535bkb.29 for ; Thu, 12 Sep 2013 08:56:29 -0700 (PDT) Message-ID: <5231E463.401@profitbricks.com> Date: Thu, 12 Sep 2013 17:57:23 +0200 From: Jack Wang MIME-Version: 1.0 References: <5231A524.2080103@profitbricks.com> <20130912135836.GE18070@stefanha-thinkpad.redhat.com> <20130912142750.GK3514@dhcp-200-207.str.redhat.com> In-Reply-To: <20130912142750.GK3514@dhcp-200-207.str.redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC] O_EXCL or not open block device List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: NeilBrown , Paolo Bonzini , Dongsu Park , Stefan Hajnoczi , qemu-devel On 09/12/2013 04:27 PM, Kevin Wolf wrote: > Am 12.09.2013 um 15:58 hat Stefan Hajnoczi geschrieben: >> On Thu, Sep 12, 2013 at 01:27:32PM +0200, Jack Wang wrote: >>> Hi all, >>> >>> We're using qemu export md-raid to guest OS, and we saw deadlock on >>> MD(which is already fixed by Neil), please see thread below: >>>> http://marc.info/?l=linux-raid&m=137894040228125&w=2 >>> >>> As Neil suggested it would be good for userspace applications to call >>> open() with O_EXCL flag, to avoid such MD hanging problems at the begining. >>> >>> And we checked qemu, it looks it doesn't include O_EXCL flag when open >>> block device. >>> >>> After search in the mail list we found there are a similar discussion: >>> >>> https://lists.nongnu.org/archive/html/qemu-devel/2011-04/msg00722.html >>> >>>> To O_EXCL or not to O_EXCL open host_cdrom >>> >>> Which looks prefer enable O_EXCL, but I checked latest qemu tree, there >>> no such code, could anyone give comments on this? >> >> Continuing from that discussion, I think the shared CD-ROM case is >> something we must avoid breaking. It worked in the past so it shouldn't >> break in a new QEMU version. >> >> Kevin: Do you think we should add an option to the host_device >> BlockDriver that sets the O_EXCL open flag? That way users and new >> libvirt can use O_EXCL for host block devices. >> >> The simpler alternative is to always use O_EXCL for non-CDROM host >> devices. Simple patch, no configuration required, but it means we >> continue to lack O_EXCL on CD-ROMs. > > I'm not sure why O_EXCL would be correct on generic block devices when > it's wrong on CD-ROMs. I think it's in fact more likely that other > devices are shared, as backing files. > > Adding an option is certainly possible, but what would the default be? > If O_EXCL is off by default, would anyone actually use it? > > Kevin > Hi Kevin, Could you elaborate more about why you think it's wrong to use O_EXCL on block device? There are already quite a lot different options in qemu, if there is such new option, people who care about it will use it I believe. Thanks Jack