[libvirt] [PATCH v5 3/3] libvirtd: fix crash on termination

Nikolay Shirokovskiy nshirokovskiy at virtuozzo.com
Fri Dec 22 08:11:36 UTC 2017

On 21.12.2017 17:59, John Ferlan wrote:
> [...]
>>> Patch looks good to me too. But still original "libvirtd: fix crash on termination"
>>> fixes another issue and if applied fixes "virt-manager issue" as well as John
>>> figured out.
> Not sure there's enough coffee in the house this morning to make me
> "awake enough" for all this stuff, especially just before a holiday
> break. But perhaps better now than after the new year... sooo....
>> Finally I'm back on track with this (sorry for it taking so long), although
>> you're right about this, it's not the correct fix, it's just a byproduct of
>> your patch, however, the whole thing about closing connections and releasing
>> memory is a bit of a mess. For example, right now, we only dispose of service
>> objs (but we don't close them, we do that in virNetServerClose), but we both
>> close and dispose of the client objs. Another thing, we toggle the service to
>> stop accepting any new connections, but that's irrelevant at that point because
>> we've closed them already by that time as a product of calling
>> virNetDaemonClose->virNetServerClose->virNetServerServiceClose - so that
> virNetServerServiceClose could call virNetServerServiceToggle(,false),
> even though it probably doesn't matter at this point.
> Makes me wonder why virNetServerUpdateServicesLocked wasn't called in
> virNetServerDispose instead of open coding the nservices loop (sigh).
>> should be removed...I drifted a bit here, anyway, we need to make a clear
>> distinction when we're releasing memory and when we're shutting down some kind
>> of service - sometimes both can be done at the same time (see below), however
>> it's very clear we can't do it here. Your issue is that Close currently only
>> closes the sockets (this reflects my point of "shutting down a service") but
>> does nothing with the threadpool connected to the server, thus leaving the
>> worker threads to continue running and executing APIs the results of which they
>> shouldn't even be able to return back to the client, since we're shutting down.
>> Now the thing with threadpool is that both memory release and teardown are
>> merged into one "object disposal" operation and therefore done as part of
>> virNetServerDispose. Since I understand a removal from the hash table as a
>> memory release operation, we should not be calling virHashRemoveAll from
>> virNetDaemonClose. Now, I see 2 options:
>> 1) split virThreadPoolFree into 2 functions, one which only broadcasts the
>> "die" message and joins the threads (or waits for them in this case...) and the
>> other releasing the resources - can't say I'm a fan of this one
> Kind of a "virThreadPoolStop" type operation...
>> 2) call virThreadPoolFree from virNetServerClose instead...
>> None of those approaches is ideal, but I can't seem to think off anything
>> better at the moment.
> I like 2 better, but it doesn't fix the problem of a long running thread
> (such as GetAllDomainStats), it just moves the cheese.  Although I have
> a feeling the virStateShutdown series Nikolay is promoting may solve
> that issue.

No, no. The problem will be fixed as I mentined in a different reply.
It just can not be solved by solely virHashRemoveAll with current
unref ordering in libvirtd.c. 

> It's really a conundrum w/r/t how much time to spend on this especially
> if the short/long term goal is a shims for libvirtd (e.g. libvirt_qemu)
> which will move the cheese even more.
> I'm going to move away from this for now, maybe a fresh look will help.
> Right now I'm not sure I can focus on this with the other threads I'm
> involved in.
> John
>> I'm open to discuss any other suggestions.
>> Erik
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