Re: pte_update and 64-bit PTEs on PPC32?

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> Ben, I agree with you about having the flags in a single word so we can 
> lock them properly.  In the short term it appears that the issue I'm 
> running into is explicit with ptep_get_and_clear():
> 
> static inline pte_t ptep_get_and_clear(struct mm_struct *mm, unsigned 
> long addr,
>                                         pte_t *ptep)
> {
>          return __pte(pte_update(ptep, ~_PAGE_HASHPTE, 0));
> }
> 
> It appears that we should be returning the pte that was passed in, 
> before its modified?  (seems a little silly to me, why bother, the 
> caller could do this 

No, we should return the PTE that was there before the change
atomically. It can't be done by the caller as if the caller was just
reading the PTE, there would be a race where the PTE could still change
between what you read and what you update. It has to be an atomic
operation.

> -- i've posted to lkml on the issue?).  Anyways, 
> since pte_update only returns the lower 32-bits the wrong thing 
> happens.  The following seems to be a better implementation of 
> ptep_get_and_clear() for ppc32 which resolves my issue:
> 
> static inline pte_t ptep_get_and_clear(struct mm_struct *mm, unsigned 
> long addr,
>                                         pte_t *ptep)
> {
>          pte_t tmp = *ptep;
>          pte_update(ptep, ~_PAGE_HASHPTE, 0);
>          return tmp;
> }
> 
> If we are ok with this I'll send a patch upstream for it.  I'd like to 
> still discuss how to make this all proper long term.  Currently, 
> ptep_get_and_clear was the only user of pte_update that used the return 
> value for anything but flags.  One change would be for it to return 
> just the flags portion of the pte it was given.  Another would be for 
> us to implement a proper 64-bit pte version of pte_update.
> 
> - kumar
> 
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-- 
Benjamin Herrenschmidt <benh@kernel.crashing.org>