blkfront problem in pvops kernel when barriers enabled

[Marek Marczykowski <marmarek@mimuw.edu.pl>]

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Hello,

Pvops block frontend (tested vanilla 3.0.3, 3.1rc2, Konrad's testing
branch) produces a lot of I/O errors when barriers are enabled but
cannot be used.

On xenlinux I've got message:
[   15.036921] blkfront: xvdb: empty write barrier op failed
[   15.036936] blkfront: xvdb: barriers disabled

and after that, everything works fine. On pvops - I/O errors.
As backend I've used 2.6.38.3 xenlinux (based on SUSE package) and
3.1rc2 with same result.

When I disable barriers (patching blkbackend to set feature-barrier=0)
everything works fine with all above versions.

My setup is xen-4.1.1 (if it matters), backends: phy from device-mapper
device and phy from loop device; frontends covered by device-mapper
snapshot, which is set up in domU initramfs.

It looks like some race condition, because when I setup device-mapper in
domU and mount it manually (which cause some delays between steps), it
works fine...

Have you idea why it happens? What additional data can I provide debug it?

In addition it should be possible to disable barrier without patching
module... Perhaps some pciback module parameter? Or leave feature-*
xenstore entries alone if present before device initialization.

-- 
Pozdrawiam / Best Regards,
Marek Marczykowski         | RLU #390519
marmarek at mimuw edu pl   | xmpp:marmarek at staszic waw pl


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