rcu: Provide diagnostic option to slow down grace-period initialization

Grace-period initialization normally proceeds quite quickly, so
that it is very difficult to reproduce races against grace-period
initialization.  This commit therefore allows grace-period
initialization to be artificially slowed down, increasing
race-reproduction probability.  A pair of new Kconfig parameters are
provided, CONFIG_RCU_TORTURE_TEST_SLOW_INIT to enable the slowdowns, and
CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY to specify the number of jiffies
of slowdown to apply.  A boot-time parameter named rcutree.gp_init_delay
allows boot-time delay to be specified.  By default, no delay will be
applied even if CONFIG_RCU_TORTURE_TEST_SLOW_INIT is set.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c5cefb3c009c..feee8dab441e 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1257,6 +1257,30 @@ config RCU_TORTURE_TEST_RUNNABLE
           Say N here if you want the RCU torture tests to start only
           after being manually enabled via /proc.
 
+config RCU_TORTURE_TEST_SLOW_INIT
+        bool "Slow down RCU grace-period initialization to expose races"
+        depends on RCU_TORTURE_TEST
+        help
+          This option makes grace-period initialization block for a
+          few jiffies between initializing each pair of consecutive
+          rcu_node structures.  This helps to expose races involving
+          grace-period initialization, in other words, it makes your
+          kernel less stable.  It can also greatly increase grace-period
+          latency, especially on systems with large numbers of CPUs.
+          This is useful when torture-testing RCU, but in almost no
+          other circumstance.
+
+          Say Y here if you want your system to crash and hang more often.
+          Say N if you want a sane system.
+
+config RCU_TORTURE_TEST_SLOW_INIT_DELAY
+        int "How much to slow down RCU grace-period initialization"
+        range 0 5
+        default 0
+        help
+          This option specifies the number of jiffies to wait between
+          each rcu_node structure initialization.
+
 config RCU_CPU_STALL_TIMEOUT
         int "RCU CPU stall timeout in seconds"
         depends on RCU_STALL_COMMON