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mm: replace ACCESS_ONCE with READ_ONCE or barriers

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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 <[email protected]>
Cc: [email protected]
Acked-by: Paul E. McKenney <[email protected]>
This commit is contained in:
Christian Borntraeger 2014-12-07 21:41:33 +01:00
commit e37c698270
3 changed files with 13 additions and 3 deletions
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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,
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))