[pve-devel] [RFC qemu-server] API/create: move locking inside worker

[Thomas Lamprecht]

Am 04/06/2018 um 01:24 PM schrieb Thomas Lamprecht:
> Am 04/06/2018 um 12:28 PM schrieb Fabian Grünbichler:
>> On Fri, Apr 06, 2018 at 11:54:03AM +0200, Thomas Lamprecht wrote:
>>> Move the locking inside worker, so that the process doing the actual
>>> work (create or restore) holds the lock, and can call functions which
>>> do locking without deadlocking.
>>>
>>> This mirrors the behaviour we use for containers, and allows to add
>>> an 'autostart' parameter which starts the VM after successful
>>> creation. vm_start needs the lock and as not the worker but it's
>>> parents held it, it couldn't know that it was actually save to
>>> continue...
>>>
>>> Signed-off-by: Thomas Lamprecht <t.lamprecht at proxmox.com>
>>> ---
>>>
>>> I discussed this with Fabian a few months ago and have something in
>>> mind that this shouldn't be that easy, but I cannot remember what
>>> exactly that reason was, so RFC. :)
>>
>> there is one issue - if somebody holds the flock and you only realize it
>> after you have forked, you did a fork for nothing (and instead of a
>> rather fast "timeout" error message, you have to check the task log.
>> this is not nice from a usability perspective, although it should not
>> cause problems from a technical/lockdep one ;)
>>
> 
> Ah, yeah, I could add a simple check if the VM is already locked before
> forking the worker, I mean that's obviously racy but should catch most
> cases, and we do that elsewhere too, AFAIK.
> 
> I'll try to add it and just send it out, if it's accetable to you then
> we could apply it as a temporary improvement :)
> 


OK, scratch it, without calling flock myself (here or in a new method) I 
cannot do a timeout-less 'check_lock', AFAIS, so better just do the
right thing (after vacantion) ;)