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



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. So if the domain
is not running you won't get more of them. The toolstack can only send
replies.

Tamas



 


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