Re: [Qemu-devel] [RFC] O_EXCL or not open block device

[Jack Wang <jinpu.wang@profitbricks.com>]

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