[libvirt] Internal error message from netcf when trying to start libvirtd

Mon Dec 2 17:55:02 UTC 2013

On Mon, Dec 02, 2013 at 06:53:37PM +0100, Michal Privoznik wrote:
> On 02.12.2013 18:40, Daniel P. Berrange wrote:
> > On Mon, Dec 02, 2013 at 06:38:37PM +0100, Michal Privoznik wrote:
> >> On 02.12.2013 18:24, Daniel P. Berrange wrote:
> >>> On Mon, Dec 02, 2013 at 06:17:00PM +0100, Michal Privoznik wrote:
> >>>> On 17.10.2013 14:40, Daniel P. Berrange wrote:
> >>>>> On Tue, Oct 15, 2013 at 06:03:27PM +0200, Christophe Fergeau wrote:
> >>>>>> Hey,
> >>>>>>
> >>>>>> Due to a configuration issue on my system, libvirtd is not starting on my
> >>>>>> system (not complaining about this!):
> >>>>>> 2013-10-15 15:40:51.024+0000: 10222: info : libvirt version: 1.1.3
> >>>>>> 2013-10-15 15:40:51.024+0000: 10222: error : virNetTLSContextCheckCertFile:117 : Cannot read CA certificate
> >>>>>> '/home/teuf/usr/etc/pki/CA/cacert.pem': No such file or directory
> >>>>>>
> >>>>>> However, before libvirtd exits, I get another error message from the netcf
> >>>>>> code, which is unexpected this time:
> >>>>>> 2013-10-15 15:49:18.361+0000: 10222: error : netcfStateCleanup:105 :
> >>>>>> internal error: Attempt to close netcf state driver already closed
> >>>>>>
> >>>>>> This message comes from the call of virStateCleanup() at the end of main()
> >>>>>> in libvirtd.c. virStateCleanup() should not be called before
> >>>>>> daemonStateInit() has been called in main.
> >>>>>> After this call, things get more ugly as daemonStateInit() calls
> >>>>>> virStateInitialize() from a thread, so there's probably a small window for
> >>>>>> virStateInitialize() and virStateCleanup() running concurrently if an error
> >>>>>> occurs between the call to daemonStateInit() and the call to
> >>>>>> virNetServerRun().
> >>>>>>
> >>>>>> I'm sending this email rather than a patch as I'm not sure what is the best
> >>>>>> way to fix it. The easy way would be for virStateCleanup() to be a noop
> >>>>>> when virStateInitialize() hasn't been called (iow remove the error message
> >>>>>> from netcfStateCleanup). However, this would leave this small race
> >>>>>> condition around (which is not that bad as it would only occurs in
> >>>>>> situations when the daemon fails to start). So another approach would be to
> >>>>>> set a vir_state_initialized boolean once the thread has called
> >>>>>> ivrStateInitialize, and only call virStateCleanup() when it's set.
> >>>>>>
> >>>>>> Or maybe there's a 3rd way to fix this?
> >>>>>>
> >>>>>> Let me know if you have any guidance into the best way to fix this,
> >>>>>
> >>>>> Yeah, I've known about this issue for a while and its a bit horrible
> >>>>> to solve. I think we probably need to have a global mutex to serialize
> >>>>> the virStateInitialize, virStateReload and virStateCleanup calls so
> >>>>> that none of them can run in parallel.
> >>>>>
> >>>>> I'd have virGlobalInit create the mutex instance, since we know that
> >>>>> is run from thread safe context.
> >>>>>
> >>>>> Then make the virState* calls hold that mutex while executing.
> >>>>>
> >>>>> We don't want virStateCleanup to skip execution if virStateInitialize
> >>>>> has failed though - every callback in virStateCleanup should be written
> >>>>> to be safe if its corresponding init function hasn't run. Just the
> >>>>> mutex serialization ought to be sufficient I think.
> >>>>>
> >>>>>
> >>>>> Daniel
> >>>>>
> >>>>
> >>>> Sorry for bringing up a dead thread, but as you both may have noticed I
> >>>> was trying to solve this too:
> >>>>
> >>>> https://www.redhat.com/archives/libvir-list/2013-November/msg01311.html
> >>>>
> >>>> And I don't think Dan, your approach is gonna work. I mean, the reason
> >>>> for running virStateInitialize in thread is - it may take ages to
> >>>
> >>> Actually no, the reason for using a thread is that the initialization
> >>> code needs to have a working event loop - so you cannot run the
> >>> virStateInitialize code from the same thread that is running the
> >>> event loop.
> >>
> >> Okay, this might be the reason too.
> >>
> >>>
> >>> You're probably thinking of the QEMU VM autostart code, whicih spawns a
> >>> separate thread to auto-start QEMU VMs to avoid delaying use of the QEMU
> >>> driver while (slow) autostart takes place.
> >>>
> >>
> >> No. I am thinking of the stateAutostart member which is called from
> >> within virStateInitialize(). In case of qemu driver it's
> >> qemuStateAutoStart() what is called, which boils down to calling
> >> qemuProcessStart over each autostart domain without spawning a separate
> >> thread for it. The thread you have in mind is used for
> >> qemuProcessReconnect() not qemuProcessStart().
> >>
> >>>> complete. Hence, serializing virState* will lead to main thread waiting
> >>>> for the virStateInitialize to complete (which may not happen, i.e. in
> >>>> case of deadlock somewhere in a driver).
> >>>>
> >>>> But I was thinking of rather refined locking. Turning each of
> >>>> vir*DriverTab into virObjectLockable(). Then, each access would require
> >>>> object to lock itself. Moreover, the virStateCleanup would unregister
> >>>> the driver. Unfortunately, it could only unregister stateful drivers.
> >>>
> >>> This race condition scenario only hits us in two cases
> >>>
> >>>  - libvirtd startup is being aborted due to some failure
> >>>    while StateInit is still running
> >>>  - libvirtd reload is triggered by SIGHUP causing StateReload
> >>>    to run
> >>
> >> I can see the third scenario: the daemon exiting the event loop, i.e. as
> >> a result of timeout or SIGINT. In this case you don't want to wait for
> >> virStateInitialize() to finish, do you?
> > 
> > If the event loop exits then in fact the virStateInitialize may never
> > complete, since it requires a functioning event loop. So in this case
> > then you cannot safely call virStateCleanup at all and we should just
> > skip it entirely and exit with trying to cleanup. The cleanup code is
> > pretty much worthless from a functional POV anyway - it is just there
> > to make valgrind happy.
> 
> Yep. I vote for removing virStateCleanup(). Although, I'm not sure if
> NWFilter is not doing something actually useful there. Other drivers
> seems to just free() allocated memory (=making valgrind happy).

NB, I didn't say remove virStateCleanup - just skip it if we know that
virStateInitialize hasn't finished yet.

Daniel
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