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51c40f62d4
The NWFilter code has as a deadlock race condition between
the virNWFilter{Define,Undefine} APIs and starting of guest
VMs due to mis-matched lock ordering.
In the virNWFilter{Define,Undefine} codepaths the lock ordering
is
1. nwfilter driver lock
2. virt driver lock
3. nwfilter update lock
4. domain object lock
In the VM guest startup paths the lock ordering is
1. virt driver lock
2. domain object lock
3. nwfilter update lock
As can be seen the domain object and nwfilter update locks are
not acquired in a consistent order.
The fix used is to push the nwfilter update lock upto the top
level resulting in a lock ordering for virNWFilter{Define,Undefine}
of
1. nwfilter driver lock
2. nwfilter update lock
3. virt driver lock
4. domain object lock
and VM start using
1. nwfilter update lock
2. virt driver lock
3. domain object lock
This has the effect of serializing VM startup once again, even if
no nwfilters are applied to the guest. There is also the possibility
of deadlock due to a call graph loop via virNWFilterInstantiate
and virNWFilterInstantiateFilterLate.
These two problems mean the lock must be turned into a read/write
lock instead of a plain mutex at the same time. The lock is used to
serialize changes to the "driver->nwfilters" hash, so the write lock
only needs to be held by the define/undefine methods. All other
methods can rely on a read lock which allows good concurrency.
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 6e5c79a1b5a8b3a23e7df7ffe58fb272aa17fbfb)
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>