mm: replace ACCESS_ONCE with READ_ONCE or barriers (e37c6982) · Commits · 戴 / test

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

+1 −1

Original line number	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);
		do {
		pud_t pud = ACCESS_ONCE(*pudp);
		pud_t pud = READ_ONCE(*pudp);

		next = pud_addr_end(addr, end);
		if (pud_none(pud))

mm/memory.c

+10 −1

Original line number	Diff line number	Diff line
		@@ -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) {

mm/rmap.c

+2 −1

Original line number	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
		* 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:

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