Re: [Xen-devel] [PATCH v2] x86/mm: Add debug code to detect illegal page_lock and put_page_type ordering

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.01.18 at 16:02, <george.dunlap@xxxxxxxxxx> wrote:
> The fix for XSA-242 depends on the same cpu never calling
> _put_page_type() while holding a page_lock() for that page; doing so
> may cause a deadlock under the right conditions.
> 
> Furthermore, even before that, there was never any discipline for the
> order in which page locks are grabbed; if there are any paths that
> grab the locks for two different pages at once, we risk creating the
> conditions for a deadlock to occur.
> 
> These are believed to be safe, because it is believed that:
> 1. No hypervisor paths ever lock two pages at once, and
> 2. We never call _put_page_type() on a page while holding its page lock.
> 
> Add a check to debug builds to catch any violations of these
> assumpitons.
> 
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> ---
> v2:
> - Make wrapper macros to get rid of ugly #ifdefs
> - Use "current_locked_page*" prefix
> - Reword commit message
> 
> NB this doesn't address Andrew's comment from v1 about adding "more
> than just a debug check".  I think we should check in the ASSERT()
> while we discuss future potential work, and not let the best become
> the enemy of the good.

I agree, and hence
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
unless Andrew really means to veto this going in.

Jan


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