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>

3 files changed, 13 insertions, 3 deletions

diff --git a/mm/gup.c b/mm/gup.c
index cd62c8c90d4a..f2305deaef50 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 3e503831e042..d86aa88902a0 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3202,7 +3202,16 @@ static int handle_pte_fault(struct mm_struct *mm,
         pte_t entry;
         spinlock_t *ptl;
 
-        entry = ACCESS_ONCE(*pte);
+        /*
+         * some architectures can have larger ptes than wordsize,
+         * e.g.ppc44x-defconfig has CONFIG_PTE_64BIT=y and CONFIG_32BIT=y,
+         * so READ_ONCE or ACCESS_ONCE cannot guarantee atomic accesses.
+         * The code below just needs a consistent view for the ifs and
+         * we later double check anyway with the ptl lock held. So here
+         * a barrier will do.
+         */
+        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 19886fb2f13a..1e542744e131 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: