On Wed, Dec 03, 2014 at 11:30:14PM +0100, Christian Borntraeger wrote: > 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 > READ_ONCE that does implicit scalar accesses. > > mm_find_pmd is tricky, because m68k and sparc(32bit) define pmd_t > as array of longs. This code requires just that the pmd_present > and pmd_trans_huge check are done on the same value, so a barrier > is sufficent. > > Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > --- > mm/gup.c | 2 +- > mm/memory.c | 2 +- > mm/rmap.c | 3 ++- > 3 files changed, 4 insertions(+), 3 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index cd62c8c..f2305de 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -917,7 +917,7 @@ 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 = READ_ONCE(*pudp); > > next = pud_addr_end(addr, end); > if (pud_none(pud)) > diff --git a/mm/memory.c b/mm/memory.c > index 3e50383..9e0c84e 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -3202,7 +3202,7 @@ static int handle_pte_fault(struct mm_struct *mm, > pte_t entry; > spinlock_t *ptl; > > - entry = ACCESS_ONCE(*pte); > + entry = READ_ONCE(*pte); > 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
