Re: [libvirt-users] Calling 'virsh' from within a hook script - avoid deadlock?
Thanks for the explanation(and warnings). Very interesting and useful feature.
Shlomi
-----Original Message-----
From: Daniel P. Berrange [mailto:berrange@redhat.com]
Sent: Thursday, November 07, 2013 11:20 AM
To: Michal Privoznik
Cc: TSADOK, Shlomi (Shlomi); libvirt-users(a)redhat.com
Subject: Re: [libvirt-users] Calling 'virsh' from within a hook script - avoid
deadlock?
On Thu, Nov 07, 2013 at 10:16:29AM +0100, Michal Privoznik wrote:
On 06.11.2013 16:17, Michal Privoznik wrote:
> On 06.11.2013 11:23, TSADOK, Shlomi (Shlomi) wrote:
>> [...]
>
> http://libvirt.org/hooks.html#recursive
>
> Do not do this.
>
> Michal
>
Maybe I should explain a bit why this is not supported. It's true that
libvirt is multithreaded. However, not in terms of multiple
write/modify accesses to a single domain. That is, when a domain is
being started up it is a modify access. Meanwhile, at some point
during the transaction the startup hook script is called. If the
script decides to start another modify transaction an deadlock occurs.
There is also a limit to the number of threads which deal with jobs, so even if the
threads are all busy, your hook will stall and potentially even deadlock if all creates
are stuck in create calls with hooks running even across different VMs.
Daniel
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