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Dion Bosschieter
92de6563c6
The `nwfilterBindingCreateXML` and `nwfilterConnectListAllNWFilters`
APIs can acquire locks on multiple instances of virNWFilterObj. There
is no guarantee they will acquire these locks in the same order as
each other. Thus there is a potential for deadlock if they run
concurrently acquiring locks on the same filter objects.
This flaw has always existed, but historically was rare, because
virNWFilterObjList previously used an array. This meant iteration
over filters had a fixed order, matching order of loading filters
into libvirt. The set of filter references would have to be just
right to expose the lock ordering deadlock.
In 8.2.0, commit c4fb52dc72 switched
to use a hash table, introducing non-determinism to the iteration
order, as hash buckets vary based on the hash seed. As such almost
any filter with references is exposed to the deadlock risk now.
It is not easy to guarantee lock ordering on the virNWFilterObj
instances, so acquiring `driverMutex` first, will serve to serialize
all lock acquisition on virNWFilterObj instances, avoiding the
deadlock scenario.
The major cost is that concurrency of the driver is significantly
reduced, with few other APIs able to run in parallel with updating
firewall rules.
A long term solution to this problem needs significant changes
* The mutex on virNWFilterObj would need to change to a R/W
lock.
* The filter instantiation/teardown process would need to split
into two phases. The first phase would resolve all the required
virNWFilterObj instances & acquire read locks, while holding
the 'driverMutex'. The second phase of running iptables/ebtables
commands would then run without driverMutex held.
* The filter define/undefine APIs would need to acquire write
locks, other APIs only read locks.
This would allow concurrency of filter instantiation/teardown
with everything except for filter defnie/undefine, which was
the original desire.
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
[DPB: rewrite commit message & add inline comment]
Signed-off-by: Dion Bosschieter <dionbosschieter@gmail.com>
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==============================
Libvirt API for virtualization
==============================
Libvirt provides a portable, long term stable C API for managing the
virtualization technologies provided by many operating systems. It
includes support for QEMU, KVM, Xen, LXC, bhyve, Virtuozzo, VMware
vCenter and ESX, VMware Desktop, Hyper-V, VirtualBox and the POWER
Hypervisor.
For some of these hypervisors, it provides a stateful management
daemon which runs on the virtualization host allowing access to the
API both by non-privileged local users and remote users.
Layered packages provide bindings of the libvirt C API into other
languages including Python, Perl, PHP, Go, Java, OCaml, as well as
mappings into object systems such as GObject, CIM and SNMP.
Further information about the libvirt project can be found on the
website:
https://libvirt.org
License
=======
The libvirt C API is distributed under the terms of GNU Lesser General
Public License, version 2.1 (or later). Some parts of the code that are
not part of the C library may have the more restrictive GNU General
Public License, version 2.0 (or later). See the files ``COPYING.LESSER``
and ``COPYING`` for full license terms & conditions.
Installation
============
Instructions on building and installing libvirt can be found on the website:
https://libvirt.org/compiling.html
Contributing
============
The libvirt project welcomes contributions in many ways. For most components
the best way to contribute is to send patches to the primary development
mailing list. Further guidance on this can be found on the website:
https://libvirt.org/contribute.html
Contact
=======
The libvirt project has two primary mailing lists:
* users@lists.libvirt.org (**for user discussions**)
* devel@lists.libvirt.org (**for development only**)
Further details on contacting the project are available on the website:
https://libvirt.org/contact.html