On Sat, Feb 22, 2020 at 02:33:10PM +0800, Longpeng (Mike) wrote:
> 在 2020/2/22 13:23, Qian Cai 写道:
> >> On Feb 21, 2020, at 10:34 PM, Longpeng(Mike) <longpeng2@xxxxxxxxxx> wrote:
> >>
> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> index dd8737a..90daf37 100644
> >> --- a/mm/hugetlb.c
> >> +++ b/mm/hugetlb.c
> >> @@ -4910,28 +4910,30 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> >> {
> >> pgd_t *pgd;
> >> p4d_t *p4d;
> >> - pud_t *pud;
> >> - pmd_t *pmd;
> >> + pud_t *pud, pud_entry;
> >> + pmd_t *pmd, pmd_entry;
> >>
> >> pgd = pgd_offset(mm, addr);
> >> - if (!pgd_present(*pgd))
> >> + if (!pgd_present(READ_ONCE(*pgd)))
> >> return NULL;
> >> p4d = p4d_offset(pgd, addr);
> >> - if (!p4d_present(*p4d))
> >> + if (!p4d_present(READ_ONCE(*p4d)))
> >> return NULL;
> >
> > What’s the point of READ_ONCE() on those two places?
> >
> As explained in the commit messages, it's for safe(e.g. avoid the compilier
> mischief). You can also find the same usage in the ARM64's huge_pte_offset() in
> arch/arm64/mm/hugetlbpage.c
I rather agree with Qian; if we need something like READ_ONCE() here,
why don't we always need it as part of pgd_present()? It seems like an
unnecessary burden for every user.
