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Re: [Xen-devel] [PATCH v4 03/15] libxl: introduce libxl_get_nr_cpus()

On 12/03/2013 05:54 PM, Ian Jackson wrote:
Dario Faggioli writes ("Re: [PATCH v4 03/15] libxl: introduce 
On mar, 2013-12-03 at 17:48 +0000, Ian Jackson wrote:
This number might be out of date as soon as it is read, won't it ?

Quite possible, yes.

So, are you suggesting that we shouldn't even allow the user to read it?
Or that I should mention that in the comment? (Or something else?)

Perhaps I didn't explain my concerns clearly enough.

I wonder what is it for ?  Isn't it difficult to use correctly ?

Dario uses it in the new version of libxl_vcpu_set_affinity() to limit what was considered an "unreachable cpu" in its warning. (v5 14/17) That is, if you set the affinity to 1111111111111111111111..., and there are only 4 pcpus, it will return 111100000..... These don't match, and yet there are no unreachable cpus. So it asks nr_cpus first, then only compares bits 1..[nr_cpus-1].

I'm not sure how often that would change, but if there was a race, I think it would just be a spurious error message (or lack thereof).


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