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

[Julien Grall <julien@xxxxxxx>]

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.

Cheers,

--
Julien Grall