[PATCHv5 0/8] ACCESS_ONCE and non-scalar accesses

[PATCHv5 0/8] ACCESS_ONCE and non-scalar accesses

[Christian Borntraeger]

As discussed on LKML http://marc.info/?i=54611D86.4040306%40de.ibm.com
ACCESS_ONCE might fail with specific compilers for non-scalar accesses.

Here is a set of patches to tackle that problem.

The first patch introduce READ_ONCE and ASSIGN_ONCE. If the data structure
is larger than the machine word size memcpy is used and a warning is emitted.
The next patches fix up all in-tree users of ACCESS_ONCE on non-scalar types.

Due to all the trouble when dealing with linux-next, I will defer the patch
that forces ACCESS_ONCE to work only on scalar types after rc1 to give it
a full spin in linux-next.

If nobody complains I will ask Linus to pull this for 3.19 next week.
The tree can be found at 

git://git.kernel.org/pub/scm/linux/kernel/git/borntraeger/linux.git linux-next

Changelog:
v4->v5:
1. READ_ONCE/ASSIGN_ONCE use x instead of p
2. linux/types.h --> uapi/linux/types.h u??-->__u?? to avoid header
   inclusion fun and compile errors
3. Actually provide data_access_exceeds_word_size.
4. also move handle_pte_fault to a barrier as there is ppc44x which has
   64bit ptes and 32bit word size. Some sanity check from a VM person
   would be good.

Cc: linux-mm@kvack.org

Christian Borntraeger (8):
  kernel: Provide READ_ONCE and ASSIGN_ONCE
  mm: replace ACCESS_ONCE with READ_ONCE or barriers
  x86/spinlock: Replace ACCESS_ONCE with READ_ONCE
  x86/gup: Replace ACCESS_ONCE with READ_ONCE
  mips/gup: Replace ACCESS_ONCE with READ_ONCE
  arm64/spinlock: Replace ACCESS_ONCE READ_ONCE
  arm/spinlock: Replace ACCESS_ONCE with READ_ONCE
  s390/kvm: REPLACE barrier fixup with READ_ONCE

 arch/arm/include/asm/spinlock.h   |  4 +--
 arch/arm64/include/asm/spinlock.h |  4 +--
 arch/mips/mm/gup.c                |  2 +-
 arch/s390/kvm/gaccess.c           | 18 ++++------
 arch/x86/include/asm/spinlock.h   |  8 ++---
 arch/x86/mm/gup.c                 |  2 +-
 include/linux/compiler.h          | 70 +++++++++++++++++++++++++++++++++++++++
 lib/Makefile                      |  2 +-
 lib/access.c                      |  8 +++++
 mm/gup.c                          |  2 +-
 mm/memory.c                       | 11 +++++-
 mm/rmap.c                         |  3 +-
 12 files changed, 108 insertions(+), 26 deletions(-)
 create mode 100644 lib/access.c

-- 
1.9.3

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[PATCH 2/8] mm: replace ACCESS_ONCE with READ_ONCE or barriers

[Christian Borntraeger]

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>
---
 mm/gup.c    |  2 +-
 mm/memory.c | 11 ++++++++++-
 mm/rmap.c   |  3 ++-
 3 files changed, 13 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..d86aa88 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 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

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