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/gup.c b/mm/gup.c
index cd62c8c..f2305de 100644 (file)
--- 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..d86aa88 100644 (file)
--- 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 19886fb..1e54274 100644 (file)
--- 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: