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    Subject[PATCHv2 02/10] kernel: Provide READ_ONCE and ASSIGN_ONCE
    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 provide READ_ONCE/ASSIGN_ONCE that will do all accesses via
    scalar types.

    Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
    ---
    include/linux/compiler.h | 34 ++++++++++++++++++++++++++++++++++
    1 file changed, 34 insertions(+)

    diff --git a/include/linux/compiler.h b/include/linux/compiler.h
    index d5ad7b1..0ff01f2 100644
    --- a/include/linux/compiler.h
    +++ b/include/linux/compiler.h
    @@ -186,6 +186,40 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
    # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__)
    #endif

    +#include <linux/types.h>
    +
    +static __always_inline void __assign_once_size(volatile void *p, void *res, int size)
    +{
    + switch (size) {
    + case 1: *(volatile u8 *)p = *(u8 *)res; break;
    + case 2: *(volatile u16 *)p = *(u16 *)res; break;
    + case 4: *(volatile u32 *)p = *(u32 *)res; break;
    +#ifdef CONFIG_64BIT
    + case 8: *(volatile u64 *)p = *(u64 *)res; break;
    +#endif
    + }
    +}
    +
    +#define ASSIGN_ONCE(val, p) \
    + ({ typeof(p) __val; __val = val; __assign_once_size(&p, &__val, sizeof(__val)); __val; })
    +
    +
    +static __always_inline void __read_once_size(volatile void *p, void *res, int size)
    +{
    + switch (size) {
    + case 1: *(u8 *)res = *(volatile u8 *)p; break;
    + case 2: *(u16 *)res = *(volatile u16 *)p; break;
    + case 4: *(u32 *)res = *(volatile u32 *)p; break;
    +#ifdef CONFIG_64BIT
    + case 8: *(u64 *)res = *(volatile u64 *)p; break;
    +#endif
    + }
    +}
    +
    +#define READ_ONCE(p) \
    + ({ typeof(p) __val; __read_once_size(&p, &__val, sizeof(__val)); __val; })
    +
    +
    #endif /* __KERNEL__ */

    #endif /* __ASSEMBLY__ */
    --
    1.9.3


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