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

[Kevin Wolf]

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