Re: Locking in xenevt.c

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Wed, Jun 06, 2007 at 11:58:20PM -0400, Jed Davis wrote:
> It seems to me that xenevt_fpoll needs to take the same spl and lock
> as xenevt_donotify and xenevt_fread to avoid races.  In particular, it
> seems that the event handler could be called after the check for an
> empty ring but before the corresponding selrecord, thus causing the
> calling thread to miss the selnotify() and sleep when it shouldn't.
> 
> I'd had a problem with domUs hanging while probing devices, which I
> eventually traced down to xenstored not returning from select even
> though the xenevt instance it was waiting on had an unread event.  I
> could unstick it by doing "call wakeup(selwait)" in ddb a few times,
> or by doing other things that use select, like disconnecting and
> reconnecting to the domU console.  (This also means that enough
> unrelated select()ing in the dom0 could mask the problem.)
> 
> The change, which appears to fix this problem for me, seems fairly
> obvious, but as I'm not very familiar with the code involved, I'm
> posting it here.

I think you're right; this part of the code needs to be protected. You patch
matches what other device drivers do in this area.
Please commit !

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--