From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54289) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VK7Pz-0008Mh-7C for qemu-devel@nongnu.org; Thu, 12 Sep 2013 09:58:51 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VK7Pt-00046o-MF for qemu-devel@nongnu.org; Thu, 12 Sep 2013 09:58:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55320) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VK7Pt-00046f-Dz for qemu-devel@nongnu.org; Thu, 12 Sep 2013 09:58:41 -0400 Date: Thu, 12 Sep 2013 15:58:36 +0200 From: Stefan Hajnoczi Message-ID: <20130912135836.GE18070@stefanha-thinkpad.redhat.com> References: <5231A524.2080103@profitbricks.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5231A524.2080103@profitbricks.com> 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: Jack Wang Cc: kwolf@redhat.com, Paolo Bonzini , NeilBrown , Dongsu Park , qemu-devel 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. Stefan