Re: [Xen-devel] [OSSTEST PATCH] guest_prepare_disk: Only do the umount if we set an env var

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Oct 21, 2019 at 02:32:15PM +0100, Ian Jackson wrote:
> This call to guest_umount_lv is here for the benefit of ad-hoc reruns
> of (eg) ts-guest-start tidy up any ad-hoc messing about (eg from
> earlier runs of ts-debian-fixup or something).  It is not needed in
> production runs.
> 
> Serendipitously, this osstest code discovered a bug in the Linux
> blkback: when tearing down, it sets the backend state to 6 before it
> has closed the underlying block devices.  This ultimately means that
> after "xl destroy" or "xl shutdown -w" there is a period when the
> guest's open handle onto its storage is still open.  This is wrong.
> 
> This detection depends on us winning a tricky race.  So it shows up in
> osstest as a very low probability heisenbug.  The bug is currently in
> all versions of Linux and causing a bit of a nuisance.
> 
> It would be best to add a proper check for this bug.  However, this is
> quite fiddly: really, it ought to be done as close to the xl command
> completion as possible, in the same ssh invocation.  That would
> involve a fair bit of plumbing and ad-hocery.  I don't think that
> would be proportionate for such a low-impact bug.
> 
> So instead in this patch I just disable this cleanup code in the
> troublesome case, unless it is explicitly requested by the user
> setting OSSTEST_GUEST_DISK_MOUNT_CLEANUP to a trueish value.  (This
> would be reasonably convenient for the ad-hoc testing that this call
> serves.)
> 
> Thanks to Roger for diagnosing the Linux kernel bug.
> 
> CC: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> CC: Jürgen Groß <jgross@xxxxxxxx>
> Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

Thanks:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

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