Re: xbd detachment

[David Young <dyoung%pobox.com@localhost>]

On Sat, Jul 25, 2009 at 10:29:03PM +0200, Jean-Yves Migeon wrote:
> David Young wrote:
>> I am experimenting with detaching xbd(4) units during shutdown.
>> xbd_xenbus_detach() hangs in this the loop, below:
>>
>>         hypervisor_mask_event(sc->sc_evtchn);
>>         event_remove_handler(sc->sc_evtchn, &xbd_handler, sc);
>>         while (xengnt_status(sc->sc_ring_gntref)) {
>>                 tsleep(xbd_xenbus_detach, PRIBIO, "xbd_ref", hz/2);
>>         }               xengnt_revoke_access(sc->sc_ring_gntref);
>>         uvm_km_free(kernel_map, (vaddr_t)sc->sc_ring.sring,
>>             PAGE_SIZE, UVM_KMF_WIRED);
>
> The dom0 is expected not to use the grant table ring during device  
> removal, and clear transfering/writing/reading states. If it does not  
> (hence, infinite loop on status), something is wrong in dom0.

Is there an event that I should watch for in the Dom0?

> What version/OS are you running as dom0? Is the vbd (dom0 side) a file  
> (used through vnd(4)), or a block device?

NetBSD 5.99.15, XEN3_DOM0 kernel.  The backing device is a file.

> > Should this routine follow some other protocol in order to close down
> > and revoke the grant_ref_t?
>
> Hardly, revocation will end in panic() (you cannot free a grant table  
> entry while there is a read/write lock acquired by the other end on the  
> referenced page).

What I mean is that xbd_xenbus_detach() may not use the correct
protocol, and that is why the transfer/write/read status does not
clear.  For example, what if xbd(4) indicates to the dom0 that it is
finished with the ring, and then queues a transfer on the ring due to a
programming mistake?

It seems to me that xbd_xenbus_detach() may not be used very much, if at
all.  Moreover, if it has ever been used before, then I think that the
Dom0, not the DomU, had initiated the device's detachment.  That may, in
fact, make a difference, even if it should not.

Dave

-- 
David Young             OJC Technologies
dyoung%ojctech.com@localhost      Urbana, IL * (217) 278-3933