5:46 p.m.
On 20/11/14 17:37 -0500, Nir Soffer wrote:
----- Original Message -----
> From: "Adam Litke" <alitke(a)redhat.com>
> To: devel(a)ovirt.org
> Cc: "Nir Soffer" <nsoffer(a)redhat.com>, "Francesco Romani"
<fromani(a)redhat.com>, "Federico Simoncelli"
> <fsimonce(a)redhat.com>, "Dan Kenigsberg" <danken(a)redhat.com>
> Sent: Thursday, November 20, 2014 9:15:33 PM
> Subject: contEIOVMs regression?
>
> Hi list,
>
> I am taking a look at Bug 1157421 [1] which describes a situation
> where VM's that are paused with an -EIO error are not automatically
> resumed after the problem with storage has been corrected. I have
> some patches [2] on gerrit that resolve the problem. Since this
> appears to be a regression I am looking at a non-intrusive way to fix
> it in the 3.5 branch. There is some disagreement on the proper way to
> fix this so I am hoping we can arrive at a solution through an open
> discussion.
>
> The main issue at hand is with the Event/Callback mechanism we use to
> call clientIF.contEIOVMs. According to my experiments and this online
> discussion [3] weakref does not work for instance methods such as
> clientIF.contEIOVMs. Our Event class uses weakref to prevent it from
> holding references to registered callback functions.
Why making event system more correct is required to tix [1]?
>
> I see two easy ways to fix the regression:
I don't follow, what is the regression?
Assuming that at some point contEIOVMs actually worked and was able to
automatically resume VMs, then we have a regression because given the
weakref problems I am describing herein there is no way that it is
working now. The only way we don't have a regression is if this code
has never worked to begin with.
> 1) Treat clientIF as a singleton class (which it is) and make
> contEIOVMs a module-level method which gets the clientIF instance
> and calls it's bound contEIOVMs method. See my patches [2] for the
> code behind this idea.
This is the wrong direction. There is only one place using that horrible
getInstance(), and it also could just create the single instance that we
need. We should remove getInstance() instead of using it in new code.
>
> 2) Allow Event to maintain a strong reference on the bound
> clientIF.contEIOVMs method. This will allow the current code to work
> as designed but will change the Event implementation to accomodate
> this specific use case. Since no one else appears to be using this
> code, it should have no functional impact.
The code is already holding a strong reference now, no change is
needed :-)
I disagree. From vdsm/storage/misc.py:
class Event(object):
...
def register(self, func, oneshot=False):
with self._syncRoot:
self._registrar[id(func)] = (weakref.ref(func), oneshot)
...
# ^^^ "He's dead Jim"
The function is converted into a weak reference. Since, in this case,
the function is an instance method, the reference is immediately dead
on arrival. I have verified this with debugging statements in my
environment.
> Are there any other ideas I'm missing? I am aware of plans
to
> refactor this code for 3.6 but I am more interested in a short-term,
> practical solution to address the current regression.
For the short term - fixing [1] - http://gerrit.ovirt.org/35369
should be enough, registering self.contEIOVMs.
If you want to make the code more correct, maybe use the same
method used by StoragePool._upgradeCallback?
Thanks, this looks like it might work.
--
Adam Litke