Re: [Xen-devel] [PATCH 00/12] libxl: fork: SIGCHLD flexibility

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Ian Campbell writes ("Re: [Xen-devel] [PATCH 00/12] libxl: fork: SIGCHLD 
flexibility"):
> On Fri, 2014-01-24 at 12:41 +0000, Ian Jackson wrote:
> >  Thread A                                             Thread B
> > X      release libvirt event loop lock
> >                                             entering libvirt event loop
> >                                        V     observe timeout is immediate
> 
> Is there nothing in this interval which deregisters, pauses, quiesces or
> otherwise prevents the timer from going off again until after the
> callback (when the lock would be reacquired and whatever was done is
> undone)?

No.  libvirt is quite happy to call the callback multiple times at
once.

I have just looked at the code in vireventpoll.c and there is nothing
that stops this being a problem.

> Given the behaviour I suggest above this would be prevented I think?

Yes, I think so.

> It doesn't seem all that likely triggering the same timeout multiple
> times in different threads simultaneously would be a deliberate design
> decision, so if the libvirt event core doesn't already prevent this
> somehow then it seems to me that this is just a bug in the event loop
> core.

Yes.

> In that case should be addressed in libvirt, and in any case the libvirt
> core folks should be involved in the discussion, so they have the
> opportunity to tell us how best, if at all, we can use the provided
> facilities and/or whether these issues are deliberate of things which
> should be fixed etc.

I'm looking at a way to fix this in libvirt.  Mail to follow ...

Ian.

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