Re: [Xen-devel] Is this a racing bug in page_make_sharable()?

[Tim Deegan <tim@xxxxxxx>]

At 22:43 +0800 on 16 Jan (1326753834), Nai Xia wrote:
> Hi Grzegorz,
> 
> As I understand, the purpose of the code in page_make_sharable()
> checking the ref count is to ensure that nobody unexpected is working
> on the page, and so we can migrate it to dom_cow, right?
> 
> ====
>     /* Check if the ref count is 2. The first from PGT_allocated, and
>      * the second from get_page_and_type at the top of this function */
>     if(page->count_info != (PGC_allocated | (2 + expected_refcnt)))
>     {
>         /* Return type count back to zero */
>         put_page_and_type(page);
>         spin_unlock(&d->page_alloc_lock);
>         return -E2BIG;
>     }
> ====
> 
> However, it seems to me that this ref check and the following page
> migration is not atomic( although the operations for type_info ref
> check seems good) i.e. it's possible that it passed this ref
> check but just before it goes to dom_cow, someone else gets this page?

Yes, there are a number of races around the p2m code; Anders
Lagar-Cavilla has been working on fixing the locking around p2m lookups
to take care of this.

Tim.

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