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

[Stefan Hajnoczi]

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