[PATCH RFC] rcutorture: Fully test lazy RCU

[Paul E. McKenney]

Currently, rcutorture bypasses lazy RCU by using call_rcu_hurry().
This works, avoiding the dreaded rtort_pipe_count WARN(), but fails to
fully test lazy RCU.  The rtort_pipe_count WARN() splats because lazy RCU
could delay the start of an RCU grace period for a full stutter period,
which defaults to only three seconds.

This commit therefore reverts the call_rcu_hurry() instances
back to call_rcu(), but, in kernels built with CONFIG_RCU_LAZY=y,
queues a workqueue handler just before the call to stutter_wait() in
rcu_torture_writer().  This workqueue handler invokes rcu_barrier(),
which motivates any lingering lazy callbacks, thus avoiding the splat.

Questions for review:

1.	Should we avoid queueing work for RCU implementations not
	supporting lazy callbacks?

2.	Should we avoid queueing work in kernels built with
	CONFIG_RCU_LAZY=y, but that were not booted with the
	rcutree.enable_rcu_lazy kernel boot parameter set?  (Note that
	this requires some ugliness to access this parameter, and must
	also handle Tiny RCU.)

3.	Does the rcu_torture_ops structure need a ->call_hurry() field,
	and if so, why?  If not, why not?

4.	Your additional questions here!

Reported-by: Saravana Kannan <saravanak@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index c7958a3f8d673..270d551d66482 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -572,7 +572,7 @@ static unsigned long rcu_no_completed(void)
 
 static void rcu_torture_deferred_free(struct rcu_torture *p)
 {
-	call_rcu_hurry(&p->rtort_rcu, rcu_torture_cb);
+	call_rcu(&p->rtort_rcu, rcu_torture_cb);
 }
 
 static void rcu_sync_torture_init(void)
@@ -619,7 +619,7 @@ static struct rcu_torture_ops rcu_ops = {
 	.poll_gp_state_exp	= poll_state_synchronize_rcu,
 	.cond_sync_exp		= cond_synchronize_rcu_expedited,
 	.cond_sync_exp_full	= cond_synchronize_rcu_expedited_full,
-	.call			= call_rcu_hurry,
+	.call			= call_rcu,
 	.cb_barrier		= rcu_barrier,
 	.fqs			= rcu_force_quiescent_state,
 	.gp_kthread_dbg		= show_rcu_gp_kthreads,
@@ -1138,7 +1138,7 @@ static void rcu_tasks_torture_deferred_free(struct rcu_torture *p)
 
 static void synchronize_rcu_mult_test(void)
 {
-	synchronize_rcu_mult(call_rcu_tasks, call_rcu_hurry);
+	synchronize_rcu_mult(call_rcu_tasks, call_rcu);
 }
 
 static struct rcu_torture_ops tasks_ops = {
@@ -1624,6 +1624,17 @@ static void do_rtws_sync(struct torture_random_state *trsp, void (*sync)(void))
 		cpus_read_unlock();
 }
 
+/*
+ * Do an rcu_barrier() to motivate lazy callbacks during a stutter
+ * pause.  Without this, we can get false-positives rtort_pipe_count
+ * splats.
+ */
+static void rcu_torture_writer_work(struct work_struct *work)
+{
+	if (cur_ops->cb_barrier)
+		cur_ops->cb_barrier();
+}
+
 /*
  * RCU torture writer kthread.  Repeatedly substitutes a new structure
  * for that pointed to by rcu_torture_current, freeing the old structure
@@ -1644,6 +1655,7 @@ rcu_torture_writer(void *arg)
 	int i;
 	int idx;
 	unsigned long j;
+	struct work_struct lazy_work;
 	int oldnice = task_nice(current);
 	struct rcu_gp_oldstate *rgo = NULL;
 	int rgo_size = 0;
@@ -1660,6 +1672,7 @@ rcu_torture_writer(void *arg)
 		stallsdone += (stall_cpu_holdoff + stall_gp_kthread + stall_cpu + 60) *
 			      HZ * (stall_cpu_repeat + 1);
 	VERBOSE_TOROUT_STRING("rcu_torture_writer task started");
+	INIT_WORK_ONSTACK(&lazy_work, rcu_torture_writer_work);
 	if (!can_expedite)
 		pr_alert("%s" TORTURE_FLAG
 			 " GP expediting controlled from boot/sysfs for %s.\n",
@@ -1888,6 +1901,8 @@ rcu_torture_writer(void *arg)
 				       !rcu_gp_is_normal();
 		}
 		rcu_torture_writer_state = RTWS_STUTTER;
+		if (IS_ENABLED(CONFIG_RCU_LAZY))
+			queue_work(system_percpu_wq, &lazy_work);
 		stutter_waited = stutter_wait("rcu_torture_writer");
 		if (stutter_waited &&
 		    !atomic_read(&rcu_fwd_cb_nodelay) &&