[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held
On Mon, Dec 7, 2020 at 12:30 PM Julien Grall <julien@xxxxxxx> wrote: > > Hi Jan, > > On 07/12/2020 15:28, Jan Beulich wrote: > > On 04.12.2020 20:15, Tamas K Lengyel 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: > >>>>> 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. > > > > Julien, > > > > does this change your view on the refcounting added by the patch > > at the root of this sub-thread? > > I still think the code is at best fragile. One example I can find is: > > -> guest_remove_page() > -> p2m_mem_paging_drop_page() > -> vm_event_put_request() > > guest_remove_page() is not always call on the current domain. So there > are possibility for vm_event_put_request() to happen on a foreign domain > and therefore wouldn't be protected by the current hypercall. > > Anyway, I don't think the refcounting should be part of the event > channel without any idea on how this would fit in fixing the VM event race. If the problematic patterns only appear with mem_paging I would suggest just removing the mem_paging code completely. It's been abandoned for several years now. Tamas
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