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>
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2
mm/gup.c
2
mm/gup.c
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@ -917,7 +917,7 @@ static int gup_pud_range(pgd_t *pgdp, unsigned long addr, unsigned long end,
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pudp = pud_offset(pgdp, addr);
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do {
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pud_t pud = ACCESS_ONCE(*pudp);
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pud_t pud = READ_ONCE(*pudp);
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next = pud_addr_end(addr, end);
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if (pud_none(pud))
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11
mm/memory.c
11
mm/memory.c
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@ -3202,7 +3202,16 @@ static int handle_pte_fault(struct mm_struct *mm,
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pte_t entry;
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spinlock_t *ptl;
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entry = ACCESS_ONCE(*pte);
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/*
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* some architectures can have larger ptes than wordsize,
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* e.g.ppc44x-defconfig has CONFIG_PTE_64BIT=y and CONFIG_32BIT=y,
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* so READ_ONCE or ACCESS_ONCE cannot guarantee atomic accesses.
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* The code below just needs a consistent view for the ifs and
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* we later double check anyway with the ptl lock held. So here
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* a barrier will do.
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*/
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entry = *pte;
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barrier();
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if (!pte_present(entry)) {
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if (pte_none(entry)) {
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if (vma->vm_ops) {
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@ -581,7 +581,8 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
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* without holding anon_vma lock for write. So when looking for a
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* genuine pmde (in which to find pte), test present and !THP together.
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*/
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pmde = ACCESS_ONCE(*pmd);
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pmde = *pmd;
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barrier();
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if (!pmd_present(pmde) || pmd_trans_huge(pmde))
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pmd = NULL;
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out:
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