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    SubjectRe: [PATCH tip/core/rcu 01/15] rcu: Add PROVE_RCU_DELAY to provoke difficult races
    On Thu, Aug 30, 2012 at 11:56:14AM -0700, Paul E. McKenney wrote:
    > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
    >
    > There have been some recent bugs that were triggered only when
    > preemptible RCU's __rcu_read_unlock() was preempted just after setting
    > ->rcu_read_lock_nesting to INT_MIN, which is a low-probability event.
    > Therefore, reproducing those bugs (to say nothing of gaining confidence
    > in alleged fixes) was quite difficult. This commit therefore creates
    > a new debug-only RCU kernel config option that forces a short delay
    > in __rcu_read_unlock() to increase the probability of those sorts of
    > bugs occurring.
    >
    > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    Reviewed-by: Josh Triplett <josh@joshtriplett.org>

    If you end up adding more such conditional race-provoking delays
    elsewhere in the code, consider creating a prove_rcu_udelay() wrapper
    to avoid multiple #ifdefs in the code.

    > ---
    > kernel/rcupdate.c | 4 ++++
    > lib/Kconfig.debug | 14 ++++++++++++++
    > 2 files changed, 18 insertions(+), 0 deletions(-)
    >
    > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
    > index 4e6a61b..29ca1c6 100644
    > --- a/kernel/rcupdate.c
    > +++ b/kernel/rcupdate.c
    > @@ -45,6 +45,7 @@
    > #include <linux/mutex.h>
    > #include <linux/export.h>
    > #include <linux/hardirq.h>
    > +#include <linux/delay.h>
    >
    > #define CREATE_TRACE_POINTS
    > #include <trace/events/rcu.h>
    > @@ -81,6 +82,9 @@ void __rcu_read_unlock(void)
    > } else {
    > barrier(); /* critical section before exit code. */
    > t->rcu_read_lock_nesting = INT_MIN;
    > +#ifdef CONFIG_PROVE_RCU_DELAY
    > + udelay(10); /* Make preemption more probable. */
    > +#endif /* #ifdef CONFIG_PROVE_RCU_DELAY */
    > barrier(); /* assign before ->rcu_read_unlock_special load */
    > if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
    > rcu_read_unlock_special(t);
    > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
    > index 2403a63..dacbbe4 100644
    > --- a/lib/Kconfig.debug
    > +++ b/lib/Kconfig.debug
    > @@ -629,6 +629,20 @@ config PROVE_RCU_REPEATEDLY
    >
    > Say N if you are unsure.
    >
    > +config PROVE_RCU_DELAY
    > + bool "RCU debugging: preemptible RCU race provocation"
    > + depends on DEBUG_KERNEL && PREEMPT_RCU
    > + default n
    > + help
    > + There is a class of races that involve an unlikely preemption
    > + of __rcu_read_unlock() just after ->rcu_read_lock_nesting has
    > + been set to INT_MIN. This feature inserts a delay at that
    > + point to increase the probability of these races.
    > +
    > + Say Y to increase probability of preemption of __rcu_read_unlock().
    > +
    > + Say N if you are unsure.
    > +
    > config SPARSE_RCU_POINTER
    > bool "RCU debugging: sparse-based checks for pointer usage"
    > default n
    > --
    > 1.7.8
    >


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