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Re: [Xen-devel] [PATCH 4/4] x86/mm: drop redundant smp_wmb() from _put_final_page_type()
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 20 Dec 2019 14:59:49 +0000
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- Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 20 Dec 2019 14:59:56 +0000
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On 20/12/2019 14:55, Jan Beulich wrote:
> 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).
The comment should be at get_page_light() then, saying "some callers
depend on this function being a full memory barrier", which will be far
more obvious for anyone who changes the behaviour.
With an adjustment along those lines, Acked-by: Andrew Cooper
<andrew.cooper3@xxxxxxxxxx>
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