[Qemu-devel] To O_EXCL or not to O_EXCL open host_cdrom

[Stefan Hajnoczi]

Amit and I were discussing the pros and cons of using O_EXCL to open
host CD-ROM devices on IRC but this discussion could benefit from more
input.

Linux block devices (like /dev/sr0 CD-ROMs) can be opened with O_EXCL
and only one userspace process will succeed at a time.  This prevents
programs from interfering with each other.  The polling daemons, hald
and udisks, use O_EXCL and mount does too.

Today QEMU does not use O_EXCL and will therefore access host CD-ROMs
while they are in use by other programs.  This also means that
programs can be started on the host while QEMU is already running that
may interfere with the virtual machine's ability to access the CD-ROM
(for example by ejecting it).

Therefore, it sounds reasonable to switch to O_EXCL to prevent
interfering with other programs and to prevent other programs
interfering with QEMU.

On the downside, it will no longer be possible to share a host CD-ROM
between multiple virtual machines or to mount it on host while passing
it through to a guest.  These scenarios are not safe because on of the
clients could eject the device, spoiling the party for everyone else.
However, it is a handy feature for putting installation media into a
machine and installing several guests at the same time.

The other concern I have about using O_EXCL is that we expose
ourselves to race conditions if there is ever a need to re-open the
device.  When QEMU closes its file descriptor another program may be
scheduled to run and open the device with O_EXCL.  Now QEMU will not
be able to open the CD-ROM anymore.  From the guest perspective this
could be at an odd time and we'd have to start failing requests.
Today we do not re-open host CD-ROMs though so this isn't a pressing
problem.

Any thoughts?  Additional pros/cons I've missed?

Stefan