Re: [patch 2/2] fix SMP data race in pagetable setup vs walking

[Hugh Dickins <hugh@veritas.com>]

On Mon, 5 May 2008, Linus Torvalds wrote:
> On Mon, 5 May 2008, Nick Piggin wrote:
> > 
> > Index: linux-2.6/include/asm-x86/pgtable_32.h
> > ===================================================================
> > --- linux-2.6.orig/include/asm-x86/pgtable_32.h
> > +++ linux-2.6/include/asm-x86/pgtable_32.h
> > @@ -133,7 +133,12 @@ extern int pmd_bad(pmd_t pmd);
> >   * pgd_offset() returns a (pgd_t *)
> >   * pgd_index() is used get the offset into the pgd page's array of pgd_t's;
> >   */
> > -#define pgd_offset(mm, address) ((mm)->pgd + pgd_index((address)))
> > +#define pgd_offset(mm, address)						\
> > +({									\
> > +	pgd_t *ret = ((mm)->pgd + pgd_index((address)));		\
> > +	smp_read_barrier_depends(); /* see mm/memory.c:__pte_alloc */	\
> > +	ret;								\
> > +})
> 
> Is there some fundamental reason this needs to be a macro?
> 
> It is really ugly, and it would be much nicer to make this an inline 
> function if at all possible.
> 
> Yeah, maybe it requires some more #include's, but ..

My recollection is that it gets difficult once you reach
pte_offset_map(), which involved kmap_atomic() (now kmap_atomic_pte()),
which gets twisty.  Or perhaps my problems arose from doing
pte_offset_map_lock() in the generic linux/mm.h: harder to
herd that safely through the different arch builds.

Hugh

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