Re: [Xen-devel] [PATCH] cpupools: retry cpupool-destroy if domain in cpupool is dying

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

On 14.05.2014 11:16, George Dunlap wrote:
> On Mon, May 12, 2014 at 12:49 PM, Juergen Gross
> <juergen.gross@xxxxxxxxxxxxxx> wrote:
> > When a cpupool is destroyed just after the last domain has been stopped the
> > domain might already be removed from the domain list without being removed
> > from the cpupool.
> > It is easy to detect this situation and to return EAGAIN in this case which
> > is already handled in libxc by doing a retry.
> OK, I hate to be picky over two lines, but it still seems to me like
> this is papering over issues instead of dealing with them properly.
> The real problem here is that "for_each_domain_in_cpupool()" doesn't
> actually go over every domain in the cpupool.  Instead of making it so
> that it actually does, you're compensating for that fact in an ad-hoc
> fashion.
>
> Now as it happens, it looks like all the other current uses of
> for_each_domain_in_cpupool() work just fine if there are domains in
> the pool it doesn't see, as long as they're about to disappear.  But
> we've already seen a bug caused because of a situation where "don't
> see domains that are about to disappear" *does* actually cause a
> problem; working around it is just setting a trap for future
> developers to fall into.  (And who knows, there may already be a bug
> we haven't discovered in the other invocations of
> for_each_domain_in_cpupool()).
This isn't unique to for_each_domain_in_cpupool(). It is a problem for all
uses of for_each_domain() which are related to resources freed only in
complete_domain_destroy().

In theory I could built a domain list for each cpupool which I could use for
for_each_domain_in_cpupool(). In this case there would be situations when
for_each_domain_in_cpupool() sees a domain which isn't seen by
for_each_domain(). Do you think this would be better? I don't.


Juergen

--
Juergen Gross                 Principal Developer Operating Systems
PSO PM&D ES&S SWE OS6                  Telephone: +49 (0) 89 62060 2932
Fujitsu                                   e-mail: juergen.gross@xxxxxxxxxxxxxx
Mies-van-der-Rohe-Str. 8                Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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