Re: [Xen-devel] [PATCH 4/4] x86/mm: drop redundant smp_wmb() from _put_final_page_type()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.12.2019 15:51, Andrew Cooper wrote:
> On 20/12/2019 14:20, Jan Beulich wrote:
>> get_page_light()'s use of cmpxchg() is a full barrier already anyway.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> While true, is this actually a clever change to make?
> 
> The implementation of get_page_light() could plausibly change and no
> longer be a full barrier, introducing a vulnerability here.  OTOH,
> smp_wmb() is free.

It's free at the CPU level, but not at the compiler one (where
it still is a barrier). I also don't think get_page_light()
could change as drastically as losing its LOCK-ed operation,
or if it did the author (and reviewer) would be well advised
to at least briefly audit use sites (one of the reasons I'm
leaving a comment).

Jan


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