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rcu: Add PROVE_RCU_DELAY to provoke difficult races

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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>
This commit is contained in:
Paul E. McKenney 2012-07-02 14:42:01 -07:00 committed by Paul E. McKenney
parent a10d206ef1
commit e3ebfb96f3
2 changed files with 18 additions and 0 deletions
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4
kernel/rcupdate.c
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@ -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);

14
lib/Kconfig.debug
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@ -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