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

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

On Fri, Dec 4, 2020 at 6:29 AM Julien Grall <julien@xen.org> 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.

Tamas