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>

3 files changed, 40 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index bfcb1a62a7b4..94de410ec341 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2968,6 +2968,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                         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
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3b7e4133ca99..b42001fd55fb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -160,6 +160,12 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
 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
@@ -1769,6 +1775,10 @@ static int rcu_gp_init(struct rcu_state *rsp)
                 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);
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