Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Fri, Dec 4, 2020 at 2:22 PM Julien Grall <julien.grall.oss@xxxxxxxxx> wrote:
>
> On Fri, 4 Dec 2020 at 19:15, Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx> 
> wrote:
> >
> > On Fri, Dec 4, 2020 at 10:29 AM Julien Grall <julien@xxxxxxx> wrote:
> > >
> > >
> > >
> > > On 04/12/2020 15:21, Tamas K Lengyel wrote:
> > > > On Fri, Dec 4, 2020 at 6:29 AM Julien Grall <julien@xxxxxxx> wrote:
> > > >>
> > > >> Hi Jan,
> > > >>
> > > >> On 03/12/2020 10:09, Jan Beulich wrote:
> > > >>> On 02.12.2020 22:10, Julien Grall wrote:
> > > >>>> On 23/11/2020 13:30, Jan Beulich wrote:
> > > >>>>> While there don't look to be any problems with this right now, the 
> > > >>>>> lock
> > > >>>>> order implications from holding the lock can be very difficult to 
> > > >>>>> follow
> > > >>>>> (and may be easy to violate unknowingly). The present callbacks 
> > > >>>>> don't
> > > >>>>> (and no such callback should) have any need for the lock to be held.
> > > >>>>>
> > > >>>>> However, vm_event_disable() frees the structures used by respective
> > > >>>>> callbacks and isn't otherwise synchronized with invocations of these
> > > >>>>> callbacks, so maintain a count of in-progress calls, for 
> > > >>>>> evtchn_close()
> > > >>>>> to wait to drop to zero before freeing the port (and dropping the 
> > > >>>>> lock).
> > > >>>>
> > > >>>> AFAICT, this callback is not the only place where the 
> > > >>>> synchronization is
> > > >>>> missing in the VM event code.
> > > >>>>
> > > >>>> For instance, vm_event_put_request() can also race against
> > > >>>> vm_event_disable().
> > > >>>>
> > > >>>> So shouldn't we handle this issue properly in VM event?
> > > >>>
> > > >>> I suppose that's a question to the VM event folks rather than me?
> > > >>
> > > >> Yes. From my understanding of Tamas's e-mail, they are relying on the
> > > >> monitoring software to do the right thing.
> > > >>
> > > >> I will refrain to comment on this approach. However, given the race is
> > > >> much wider than the event channel, I would recommend to not add more
> > > >> code in the event channel to deal with such problem.
> > > >>
> > > >> Instead, this should be fixed in the VM event code when someone has 
> > > >> time
> > > >> to harden the subsystem.
> > > >
> > > > I double-checked and the disable route is actually more robust, we
> > > > don't just rely on the toolstack doing the right thing. The domain
> > > > gets paused before any calls to vm_event_disable. So I don't think
> > > > there is really a race-condition here.
> > >
> > > The code will *only* pause the monitored domain. I can see two issues:
> > >     1) The toolstack is still sending event while destroy is happening.
> > > This is the race discussed here.
> > >     2) The implement of vm_event_put_request() suggests that it can be
> > > called with not-current domain.
> > >
> > > I don't see how just pausing the monitored domain is enough here.
> >
> > Requests only get generated by the monitored domain.
>
> If that's the case, then why is vm_event_put_request() able to
> deal with a non-current domain?
>
> I understand that you are possibly trusting who may call it, but this
> looks quite fragile.

I didn't write the system. You probably want to ask that question from
the original author.

Tamas