mm: replace ACCESS_ONCE with READ_ONCE or barriers
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 for the gup code.
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.
A similar case is in handle_pte_fault. On ppc44x the word size is
32 bit, but a pte is 64 bit. A barrier is ok as well.
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: linux-mm@kvack.org
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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 @@
* 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: