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Date: Mon, 31 Aug 2020 11:17:52 -0700 From: paulmck@...nel.org To: linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com, kernel-team@...com, mingo@...nel.org Cc: elver@...gle.com, andreyknvl@...gle.com, glider@...gle.com, dvyukov@...gle.com, cai@....pw, boqun.feng@...il.com, "Paul E . McKenney" <paulmck@...nel.org> Subject: [PATCH kcsan 06/19] kcsan: Skew delay to be longer for certain access types From: Marco Elver <elver@...gle.com> 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]. Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org> Signed-off-by: Marco Elver <elver@...gle.com> Signed-off-by: Paul E. McKenney <paulmck@...nel.org> --- 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 4c8b40b..95a364e 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); } @@ -470,7 +474,7 @@ 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 -- 2.9.5
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