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 <[e-mail]>

Documentation/kernel-parameters.txt

···
 			Set maximum number of finished RCU callbacks to
 			process in one batch.
 
+	rcutree.gp_init_delay=	[KNL]
+			Set the number of jiffies to delay each step of
+			RCU grace-period initialization.  This only has
+			effect when CONFIG_RCU_TORTURE_TEST_SLOW_INIT is
+			set.
+
 	rcutree.rcu_fanout_leaf= [KNL]
 			Increase the number of CPUs assigned to each
 			leaf rcu_node structure.  Useful for very large

kernel/rcu/tree.c

···
 static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
 module_param(kthread_prio, int, 0644);
 
+/* Delay in jiffies for grace-period initialization delays. */
+static int gp_init_delay = IS_ENABLED(CONFIG_RCU_TORTURE_TEST_SLOW_INIT)
+				? CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY
+				: 0;
+module_param(gp_init_delay, int, 0644);
+
 /*
  * Track the rcutorture test sequence number and the update version
  * number within a given test.  The rcutorture_testseq is incremented
···
 		raw_spin_unlock_irq(&rnp->lock);
 		cond_resched_rcu_qs();
 		ACCESS_ONCE(rsp->gp_activity) = jiffies;
+		if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_SLOW_INIT) &&
+		    gp_init_delay > 0 &&
+		    !(rsp->gpnum % (rcu_num_nodes * 10)))
+			schedule_timeout_uninterruptible(gp_init_delay);
 	}
 
 	mutex_unlock(&rsp->onoff_mutex);

lib/Kconfig.debug

···
 	  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