Re: [PATCH tip/core/rcu 01/15] rcu: Add PROVE_RCU_DELAY to provoke difficult races

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.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.

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
> > 
>