[libvirt] [PATCH] qemu: lock: unlock vm during qemuBuildCommandLine

[Daniel P. Berrange]

On Thu, Apr 23, 2015 at 07:00:21PM +0800, zhang bo wrote:
> The reason for the problem is that:
> 1 guestA locks vm while creating each tapDev(virNetDevTapCreate) in qemuBuildCommandLine(), for 10seconds
> 2 guestB calls qemuMigrationPrepareAny->*virDomainObjListAdd* to get its vm object, which locks 'doms'
>   and waits for the vm lock.
> 3 doms will be locked until guestA unlock its vm, we say that's 10 seconds.
> 4 guestC calls qemuDomainMigrateFinish3->virDomainObjListFindByName, which tries to lock doms. because it's

Ok, this is the real core problem - FindByName has a bad impl that requires
iterating over every single guest. Unfortunately due to the design of the
migration API we can't avoid this call, but we could add a second hash table
of name -> virDomainObj so we make it O(1) and lock-less.

>   now locked by guestB, guestC blocks here, and it can't be unpaused for at least 10 seconds.
> 5 then comes to guestD, guestE, guestF, etc, the downtime will be added up, to even 50 seconds or more.
> 6 the command 'virsh list' is blocked as well.
> 
> Thus, we think the problem must be solved.

Regards,
Daniel
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