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    Subject[PATCH v2 3/8] kcsan: Skew delay to be longer for certain access types
    From
    For compound instrumentation and assert accesses, skew the watchpoint
    delay to be longer if randomized. This is useful to improve race
    detection for such accesses.

    For compound accesses we should increase the delay as we've aggregated
    both read and write instrumentation. By giving up 1 call into the
    runtime, we're less likely to set up a watchpoint and thus less likely
    to detect a race. We can balance this by increasing the watchpoint
    delay.

    For assert accesses, we know these are of increased interest, and we
    wish to increase our chances of detecting races for such checks.

    Note that, kcsan_udelay_{task,interrupt} define the upper bound delays.
    When randomized, delays are uniformly distributed between [0, delay].
    Skewing the delay does not break this promise as long as the defined
    upper bounds are still adhered to. The current skew results in delays
    uniformly distributed between [delay/2, delay].

    Signed-off-by: Marco Elver <elver@google.com>
    ---
    v2:
    * Commit message rewording.
    ---
    kernel/kcsan/core.c | 10 +++++++---
    1 file changed, 7 insertions(+), 3 deletions(-)

    diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
    index fb52de2facf3..4633baebf84e 100644
    --- a/kernel/kcsan/core.c
    +++ b/kernel/kcsan/core.c
    @@ -283,11 +283,15 @@ static __always_inline bool kcsan_is_enabled(void)
    return READ_ONCE(kcsan_enabled) && get_ctx()->disable_count == 0;
    }

    -static inline unsigned int get_delay(void)
    +static inline unsigned int get_delay(int type)
    {
    unsigned int delay = in_task() ? kcsan_udelay_task : kcsan_udelay_interrupt;
    + /* For certain access types, skew the random delay to be longer. */
    + unsigned int skew_delay_order =
    + (type & (KCSAN_ACCESS_COMPOUND | KCSAN_ACCESS_ASSERT)) ? 1 : 0;
    +
    return delay - (IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
    - prandom_u32_max(delay) :
    + prandom_u32_max(delay >> skew_delay_order) :
    0);
    }

    @@ -449,7 +453,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
    * Delay this thread, to increase probability of observing a racy
    * conflicting access.
    */
    - udelay(get_delay());
    + udelay(get_delay(type));

    /*
    * Re-read value, and check if it is as expected; if not, we infer a
    --
    2.28.0.rc0.142.g3c755180ce-goog
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