Re: return value of ptep_get_and_clear

[Kumar Gala]

On Apr 6, 2005, at 12:09 PM, Christoph Lameter wrote:

> On Thu, 7 Apr 2005, Nick Piggin wrote:
>
> > Kumar Gala wrote:
>  > > ptep_get_and_clear has a signature that looks something like:
> > >
>  > > static inline pte_t ptep_get_and_clear(struct mm_struct *mm, 
> unsigned
> > > long addr,
>  > >                                        pte_t *ptep)
>  > >
>  > > It appears that its suppose to return the pte_t pointed to by ptep
>  > > before its modified.  Why do we bother doing this?  The caller 
> seems
>  > > perfectly able to dereference ptep and hold on to it.  Am I 
> missing
>  > > something here?
>  > >
>  >
>  > You need to be able to *atomically* clear the pte and retrieve the
>  > old value.
>
> The effect of the clearing is that the present bit is cleared which 
> makes
>  the CPU generate a fault if this pte is referenced.
>
> The problem with replacing pte values is that the code executing is 
> racing
>  with cpu mmu access to the pte (which may set bits on i386 I 
> believe). So
>  if you would access the pte and then clear it later then there would 
> be a
>  small window where the MMU could modify the pte. These changes would 
> not
>  be detected since you later overwrite the pte.
>
> Using ptep_get_and_clear insures that this does not happen...

Thanks, I was guessing that getting the value atomically was why this 
was done after I give it a bit more thought.

- kumar

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