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    Subject[PATCH/RFC 4/7] x86: Replace ACCESS_ONCE in gup with a barrier
    Date
    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 use a barrier instead of ACCESS_ONCE to avoid that issue.

    Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
    ---
    arch/x86/mm/gup.c | 7 +++++--
    1 file changed, 5 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
    index 207d9aef..35991f6 100644
    --- a/arch/x86/mm/gup.c
    +++ b/arch/x86/mm/gup.c
    @@ -14,8 +14,12 @@

    static inline pte_t gup_get_pte(pte_t *ptep)
    {
    + pte_t pte;
    +
    #ifndef CONFIG_X86_PAE
    - return ACCESS_ONCE(*ptep);
    + pte = *ptep;
    + barrier();
    + return pte;
    #else
    /*
    * With get_user_pages_fast, we walk down the pagetables without taking
    @@ -49,7 +53,6 @@ static inline pte_t gup_get_pte(pte_t *ptep)
    * very careful -- it does not atomically load the pte or anything that
    * is likely to be useful for you.
    */
    - pte_t pte;

    retry:
    pte.pte_low = ptep->pte_low;
    --
    1.9.3


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