On Fri, Aug 31, 2012 at 09:49:36AM -0700, Josh Triplett wrote:
> On Thu, Aug 30, 2012 at 11:56:14AM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul.mcken...@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.mcken...@linaro.org>
> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
>
> Reviewed-by: Josh Triplett <j...@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.
Good point! In fact, I have added this to my list.
Thanx, Paul
> > ---
> > 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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