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Re: [libvirt] [PATCH] Fix deadlock in QEMU close callback APIs



On Thu, Feb 28, 2013 at 01:33:31PM +0000, Daniel P. Berrange wrote:
> From: "Daniel P. Berrange" <berrange redhat com>
> 
> There is a lock ordering problem in the QEMU close callback
> APIs.
> 
> When starting a guest we have a lock on the VM. We then
> set a autodestroy callback, which acquires a lock on the
> close callbacks.
> 
> When running auto-destroy, we obtain a lock on the close
> callbacks, then run each callbacks - which obtains a lock
> on the VM.
> 
> This causes deadlock if anyone tries to start a VM, while
> autodestroy is taking place.
> 
> The fix is to do autodestroy in 2 phases. First obtain
> all the callbacks and remove them from the list under
> the close callback lock. Then invoke each callback
> from outside the close callback lock.
> 
> Signed-off-by: Daniel P. Berrange <berrange redhat com>
> ---
>  src/qemu/qemu_conf.c  | 106 ++++++++++++++++++++++++++++++++++++++++++--------
>  src/util/virnetlink.c |   5 ++-
>  2 files changed, 92 insertions(+), 19 deletions(-)

Incidentally this patch is also a huge performance win. Previously
once autodestroy starts running it blocks all startup/shutdown of
VMs. When a single client had created 1000 VMs, this blocked libvirtd
for a very long time indeed. With this change we get full parallelism
in auto-destroy since only 1 VM at a time is locked, and other VMs
can continue to start/stop

Daniel
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