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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Wed, May 14, 2014 at 10:48 AM, George Dunlap
<George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Wed, May 14, 2014 at 10:16 AM, George Dunlap
> <George.Dunlap@xxxxxxxxxxxxx> 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()).
>
> Really this seems like a race in our rcu implementation wrt the domain
> list.  It seems like ideally, if you grab the domlist_read_lock, you
> should either get the domain on the list, or the domain off the list
> *and* complete_domain_destroy() completed...

Sorry, should cc' the relevant maintainers for this question...

 -George

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