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    Subject[PATCH -tip v2 08/11] READ_ONCE, WRITE_ONCE: Remove data_race() and unnecessary checks
    From
    The volatile accesses no longer need to be wrapped in data_race(),
    because we require compilers that emit instrumentation distinguishing
    volatile accesses. Consequently, we also no longer require the explicit
    kcsan_check_atomic*(), since the compiler emits instrumentation
    distinguishing the volatile accesses. Finally, simplify
    __READ_ONCE_SCALAR and remove __WRITE_ONCE_SCALAR.

    Signed-off-by: Marco Elver <elver@google.com>
    ---
    v2:
    * Remove unnecessary kcsan_check_atomic*() in *_ONCE.
    * Simplify __READ_ONCE_SCALAR and remove __WRITE_ONCE_SCALAR. This
    effectively restores Will Deacon's pre-KCSAN version:
    https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/tree/include/linux/compiler.h?h=rwonce/cleanup#n202
    ---
    include/linux/compiler.h | 13 ++-----------
    1 file changed, 2 insertions(+), 11 deletions(-)

    diff --git a/include/linux/compiler.h b/include/linux/compiler.h
    index 17c98b215572..7444f026eead 100644
    --- a/include/linux/compiler.h
    +++ b/include/linux/compiler.h
    @@ -228,9 +228,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,

    #define __READ_ONCE_SCALAR(x) \
    ({ \
    - typeof(x) *__xp = &(x); \
    - __unqual_scalar_typeof(x) __x = data_race(__READ_ONCE(*__xp)); \
    - kcsan_check_atomic_read(__xp, sizeof(*__xp)); \
    + __unqual_scalar_typeof(x) __x = __READ_ONCE(x); \
    smp_read_barrier_depends(); \
    (typeof(x))__x; \
    })
    @@ -246,17 +244,10 @@ do { \
    *(volatile typeof(x) *)&(x) = (val); \
    } while (0)

    -#define __WRITE_ONCE_SCALAR(x, val) \
    -do { \
    - typeof(x) *__xp = &(x); \
    - kcsan_check_atomic_write(__xp, sizeof(*__xp)); \
    - data_race(({ __WRITE_ONCE(*__xp, val); 0; })); \
    -} while (0)
    -
    #define WRITE_ONCE(x, val) \
    do { \
    compiletime_assert_rwonce_type(x); \
    - __WRITE_ONCE_SCALAR(x, val); \
    + __WRITE_ONCE(x, val); \
    } while (0)

    #ifdef CONFIG_KASAN
    --
    2.26.2.761.g0e0b3e54be-goog
    \
     
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