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Re: [libvirt] [PATCH] qemu: Allow graceful domain destroy

On Sat, Aug 20, 2011 at 01:06:10PM +0200, Michal Privoznik wrote:
> This patch introduces support for domain destroying via 'quit' monitor
> command which gives qemu time to flush caches and therefore prevent
> disks corruption. However, qemu can be unresponsive and to prevent
> waiting indefinitely, execute command in a separate thread and wait
> reasonable time (QEMU_QUIT_WAIT_SECONDS) on a condition. If we hit
> timeout, qemu is qemuProcessKill'ed which causes monitor close and
> therefore also thread being terminable.
> The synchronization between qemu driver and monitor-quit thread is done
> through mutex and condition. However, this alone is not enough. If a
> condition is signalized but without anybody listening signal is lost. To
> prevent this a boolean variable is used that is set iff nobody is
> listening but condition would be signalized, or iff driver is waiting on
> given condition.

If we want to talk to the monitor, then we can't do that in the
virDomainDestroy API call.

Your previous patches add a separate high priority queue to
libvirtd, for dispatch of RPC calls which do *not* use the monitor
which we need to always be processed immediately. virDomainDestroy
is one of those high priority calls. We can't do routing of the RPC
call based on flag values for the API, therefore, we must *never* use
the monitor from virDomainDestroy.

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