On Mon, Jun 24, 2013 at 11:08:16AM +0200, Stefan Bader wrote:
So this is one way I am able to get this issue resolved. It might not be ideal as maybe there is a slight chance of inconsistencies. But at least it does not lock up anymore.
-Stefan
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From f0c28a9f7af84a01bb2c3d71067adefb92e919d9 Mon Sep 17 00:00:00 2001 From: Stefan Bader <stefan.bader@canonical.com> Date: Mon, 24 Jun 2013 09:30:20 +0200 Subject: [PATCH] xen: Avoid deadlock in xenUnifiedDomainGetXMLDesc
Commit 95e18efd added the use virDomainDefPtr to several Xen driver methods. That structure is obtained by calling xenGetDomainDefForDom() which will acquire the mutex to the priv pointer. Unfortunately (at least) xenUnifiedDomainGetXMLDesc already takes that mutex and now is locking up while calling into xenDomainUsedCpus().
xenDomainUsedCpus ... nb_vcpu = xenUnifiedDomainGetMaxVcpus(dom); return xenUnifiedDomainGetVcpusFlags(...) ... if (!(def = xenGetDomainDefForDom(dom))) return xenGetDomainDefForUUID(dom->conn, dom->uuid); ... ret = xenHypervisorLookupDomainByUUID(conn, uuid); ... xenUnifiedLock(priv); name = xenStoreDomainGetName(conn, id); xenUnifiedUnlock(priv); ... if ((ncpus = xenUnifiedDomainGetVcpus(dom, cpuinfo, nb_vcpu, ... if (!(def = xenGetDomainDefForDom(dom))) [again like above]
The deadlock is observable when connecting to a Xen host using the old xm stack and trying to obtain domain XML data from any running guest (like "dumpxml 0").
This would be a minimal fix that tries to avoid the deadlock. I am not completely sure this is not allowing a small window for getting inconsistent data.
The root cause is that one public API is calling into another public API. This is bad practice in general - as well as the deadlock you see, it will result in multiple access control checks which should not happen. I'm looking into how to refactor the code to simplify the calling pattern here. Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|