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SubjectRe: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW
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On 1/25/2021 1:55 PM, Borislav Petkov wrote:
> On Mon, Jan 25, 2021 at 01:27:51PM -0800, Yu, Yu-cheng wrote:
>>> Maybe I'm missing something but those two can happen outside of the
>>> loop, no? Or is *ptep somehow changing concurrently while the loop is
>>> doing the CMPXCHG and you need to recreate it each time?
>>>
>>> IOW, you can generate upfront and do the empty loop...
>>
>> *ptep can change concurrently.
>
> Care to elaborate?
>

For example, when a thread reads a W=1, D=0 PTE and before changing it
to W=0,D=0, another thread could have written to the page and the PTE is
W=1, D=1 now. When try_cmpxchg() detects the difference, old_pte is
read again.

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