ACCESS_ONCE does not work reliably on non-scalar types. For example gcc 4.6 and 4.7 might remove the volatile tag for such accesses during the SRA (scalar replacement of aggregates) step (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145) Let's change the code to access the page table elements with a barrier afterwards. Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx> --- mm/gup.c | 4 +++- mm/memory.c | 3 ++- mm/rmap.c | 3 ++- 3 files changed, 7 insertions(+), 3 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index cd62c8c..e44af3c 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -917,7 +917,9 @@ static int gup_pud_range(pgd_t *pgdp, unsigned long addr, unsigned long end, pudp = pud_offset(pgdp, addr); do { - pud_t pud = ACCESS_ONCE(*pudp); + pud_t pud = *pudp; + + barrier(); next = pud_addr_end(addr, end); if (pud_none(pud)) diff --git a/mm/memory.c b/mm/memory.c index 3e50383..d982e35 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3202,7 +3202,8 @@ static int handle_pte_fault(struct mm_struct *mm, pte_t entry; spinlock_t *ptl; - entry = ACCESS_ONCE(*pte); + entry = *pte; + barrier(); if (!pte_present(entry)) { if (pte_none(entry)) { if (vma->vm_ops) { diff --git a/mm/rmap.c b/mm/rmap.c index 19886fb..1e54274 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -581,7 +581,8 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address) * without holding anon_vma lock for write. So when looking for a * genuine pmde (in which to find pte), test present and !THP together. */ - pmde = ACCESS_ONCE(*pmd); + pmde = *pmd; + barrier(); if (!pmd_present(pmde) || pmd_trans_huge(pmde)) pmd = NULL; out: -- 1.9.3
