From e37c698270633327245beb0fbd8699db8a4b65b4 Mon Sep 17 00:00:00 2001 From: Christian Borntraeger Date: Sun, 7 Dec 2014 21:41:33 +0100 Subject: 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 Cc: linux-mm@kvack.org Acked-by: Paul E. McKenney --- mm/memory.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) (limited to 'mm/memory.c') 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) { -- cgit v1.2.3-70-g09d2