[MirageOS-devel] Xenstore_ring deadlocks in Mirage Xenstore

[James Bielman <jamesjb@xxxxxxxxxx>]

Hi all,

I'm doing some testing of our Xenstore/Flask patches and I'm noticing an
intermittent problem with the interdomain communication locking up.

We are using "ocaml-xenstore-xen" forked from:

  https://github.com/djs55/ocaml-xenstore-xen

into our "xenstore_mac" branch at:

  https://github.com/GaloisInc/ocaml-xenstore-xen

I'm starting the Xenstore kernel with the "init-xenstore-domain" tool.

Most of the time, this works perfectly, but occasionally Xenstore sees
an event notification that data is available for reading, but the
Xenstore ring contains no data (it seems to always contain all zeros).

The problem doesn't seem to be with event signalling, because I can
replace the "block" with a "sleep" and it repeatedly sleeps reading an
empty queue.

It feels like a race condition to me, perhaps related to the fact that
dom0 is writing to the Xenstore ring before the Xenstore domain is
unpaused (during IOCTL_XENBUS_BACKEND_SETUP).

Has anyone encountered this issue before, or have any insight into
tracking it down?

Thanks,
James

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