rcu: Add PROVE_RCU_DELAY to provoke difficult races

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>

2 files changed, 18 insertions, 0 deletions

diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 4e6a61b15e86..29ca1c6da594 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 2403a63b5da5..dacbbe4d7a80 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