Re: [PATCH v17 11/26] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

[Yu, Yu-cheng]

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.