Following the links, the full comment text is:

  /* On Xen the line below does not always work. Needs investigating! */

So this is in theory a lower priority issue for the mainline kernel. Especially since it apparently hasn't caused problems for ~ 8-9 years during which Xen and Linux have been thoroughly put to the test (e.g. Amazon AWS).

Could be a "tearing out your hair" desperate sort of thing where you cast around for any possible cause....