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Commit e37c6982 authored by Christian Borntraeger's avatar Christian Borntraeger
Browse files

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: default avatarChristian Borntraeger <borntraeger@de.ibm.com>
Cc: linux-mm@kvack.org
Acked-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 230fa253

mm/gup.c

+1 −1
Original line number Original line Diff line number Diff line
@@ -917,7 +917,7 @@ static int gup_pud_range(pgd_t *pgdp, unsigned long addr, unsigned long end, 




	pudp = pud_offset(pgdp, addr);

	pudp = pud_offset(pgdp, addr);

	do {

	do {

		pud_t pud = ACCESS_ONCE(*pudp);

		pud_t pud = READ_ONCE(*pudp);





		next = pud_addr_end(addr, end);

		next = pud_addr_end(addr, end);

		if (pud_none(pud))

		if (pud_none(pud))

mm/memory.c

+10 −1
Original line number Original line Diff line number Diff line
@@ -3202,7 +3202,16 @@ static int handle_pte_fault(struct mm_struct *mm, 
	pte_t entry;

	pte_t entry;

	spinlock_t *ptl;

	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_present(entry)) {

		if (pte_none(entry)) {

		if (pte_none(entry)) {

			if (vma->vm_ops) {

			if (vma->vm_ops) {

mm/rmap.c

+2 −1
Original line number Original line Diff line number Diff line
@@ -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

	 * without holding anon_vma lock for write.  So when looking for a

	 * genuine pmde (in which to find pte), test present and !THP together.

	 * 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))

	if (!pmd_present(pmde) || pmd_trans_huge(pmde))

		pmd = NULL;

		pmd = NULL;

out:

out: