Re: [Xen-devel] [PATCH v1 03/10] libxl: store a copy of vanilla domain configuration when creating domain

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Wei Liu writes ("Re: [PATCH v1 03/10] libxl: store a copy of vanilla domain 
configuration when creating domain"):
> On Fri, Jul 25, 2014 at 04:01:06PM +0100, Ian Jackson wrote:
> > During that time various operations would block unreasonably.
> 
> That's a bit unfortunate. But what can we do to close that window?
> I think no good will come out if I try to fiddle with domain state
> during creation anyway, so locking like this may be acceptable?

No, because it might block a management daemon.  You're not allowed to
hold a lock which libxl operations will block on while doing anything
`slow' in the ao sense.  Conceivably in a sufficiently complicated
system it might even result in deadlock.

I think the right answer is for those attempts to fiddle with the
domain state to fail.

> > Also, I think you mustn't use an fcntl lock across ao operation
> > callback chains.  fcntl locks do not exclude other threads in the same
> > process.
> 
> Then we need both mutex and file lock? Mutex to protect against threads
> in the same process while file lock protect against other processes.

I think the answer is that you have to not retain the lock for so
long.

Ian.

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