Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset()

[Qian Cai]

> 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;

Whatâs the point of READ_ONCE() on those two places?