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
