[systemd-devel] cdrom_id opens device with O_EXCL, why?

[David Herrmann]

Hi

On Thu, Sep 18, 2014 at 8:22 AM, Hoyer, Marko (ADITG/SW2)
<mhoyer at de.adit-jv.com> wrote:
> Hello together,
>
> I recently stumbled over cdrom_id opening the device with the O_EXCL flag set, if it is not currently mounted:
>
> "fd = open(node, O_RDONLY|O_NONBLOCK|(is_mounted(node) ? 0 : O_EXCL));"
>
> The effect of this is that automatically mounting a cdrom sometimes results in "resource busy", if "change" uevents of the devices are processed by udevd while the automounter (udisks or something different in my case) is currently trying to mount the device triggered by a previous "add" or "change" uevent.
>
> I've to questions to this issue. Maybe someone of you can help me:
>
> 1. Is there any particular reason why cdrom_id should open the device exclusively (especially since it is not opened exclusively when it is already mounted)?
>
> 2. If there is any good reason to keep this behavior: How is the best way for an automounter to deal with this? Retry? Something different?

This was introduced in:

commit 38a3cde11bc77af49a96245b8a8a0f2b583a344c
Author: Kay Sievers <kay.sievers at vrfy.org>
Date:   Thu Mar 18 11:14:32 2010 +0100

    cdrom_id: open non-mounted optical media with O_EXCL

    This should prevent confusing drives during CD burning sessions. Based
    on a patch from Harald Hoyer.


According to this commit we use O_EXCL to not confuse burning-sessions
that might run in parallel. Admittedly, burning-sessions should
usually open the device themselves via O_EXCL, or lock critical
sections via scsi, but I guess back in the days we had to work around
bugs in other programs (or I may be missing something non-obvious; I'm
not that much into scsi..).

Regarding what do to: If you deal with cdrom devices and get EBUSY,
I'd simply ignore the device. In case of udev, you get a uevent once
udev is done, and usually that means the device changed in some way so
it's *good* you wait for it to be processed. So your automounter will
get a uevent once udev is done and can then try to mount the device.

Thanks
David