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authorChristian Borntraeger <borntraeger@de.ibm.com>2014-12-07 21:41:33 +0100
committerChristian Borntraeger <borntraeger@de.ibm.com>2014-12-18 09:54:37 +0100
commite37c698270633327245beb0fbd8699db8a4b65b4 (patch)
treeed83173785e55c3f91467a19d8c4033e723a98e9
parent230fa253df6352af12ad0a16128760b5cb3f92df (diff)
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>
-rw-r--r--mm/gup.c2
-rw-r--r--mm/memory.c11
-rw-r--r--mm/rmap.c3
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: