rcutorture: Add self-propagating callback to forward-progress testing

If rcutorture is run on a quiet system with the rcutorture.stutter module
parameter set high, then there can legitimately be an extended period
during which no RCU forward progress takes place.  This can result
in false-positive no-forward-progress splats.  This commit therefore
makes rcu_torture_fwd_prog() create a self-propagating RCU callback
to ensure that grace periods are in progress for the duration of the
forward-progress test.

Note that the RCU flavor under test must define ->call(), ->sync(),
and ->cb_barrier() for this self-propagating callback to be created.
If one or more of those rcu_torture_ops fields are NULL, then the
rcu_torture_fwd_prog() function will silently proceed without creating
the self-propagating callback.

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

kernel/rcu/rcutorture.c

@@ -1670,20 +1670,49 @@ static int __init rcu_torture_stall_init(void)
 	return torture_create_kthread(rcu_torture_stall, NULL, stall_task);
 }
 
+/* State structure for forward-progress self-propagating RCU callback. */
+struct fwd_cb_state {
+	struct rcu_head rh;
+	int stop;
+};
+
+/*
+ * Forward-progress self-propagating RCU callback function.  Because
+ * callbacks run from softirq, this function is an implicit RCU read-side
+ * critical section.
+ */
+static void rcu_torture_fwd_prog_cb(struct rcu_head *rhp)
+{
+	struct fwd_cb_state *fcsp = container_of(rhp, struct fwd_cb_state, rh);
+
+	if (READ_ONCE(fcsp->stop)) {
+		WRITE_ONCE(fcsp->stop, 2);
+		return;
+	}
+	cur_ops->call(&fcsp->rh, rcu_torture_fwd_prog_cb);
+}
+
 /* Carry out grace-period forward-progress testing. */
 static int rcu_torture_fwd_prog(void *args)
 {
 	unsigned long cver;
+	struct fwd_cb_state fcs = { .stop = 0 };
 	unsigned long gps;
 	int idx;
 	int sd;
 	int sd4;
+	bool selfpropcb = false;
 	unsigned long stopat;
 	bool tested = false;
 	int tested_tries = 0;
 	static DEFINE_TORTURE_RANDOM(trs);
 
 	VERBOSE_TOROUT_STRING("rcu_torture_fwd_progress task started");
+	if  (cur_ops->call && cur_ops->sync && cur_ops->cb_barrier) {
+		init_rcu_head_on_stack(&fcs.rh);
+		cur_ops->call(&fcs.rh, rcu_torture_fwd_prog_cb);
+		selfpropcb = true;
+	}
 	do {
 		schedule_timeout_interruptible(fwd_progress_holdoff * HZ);
 		cver = READ_ONCE(rcu_torture_current_version);
@@ -1708,6 +1737,13 @@ static int rcu_torture_fwd_prog(void *args)
 		/* Avoid slow periods, better to test when busy. */
 		stutter_wait("rcu_torture_fwd_prog");
 	} while (!torture_must_stop());
+	if (selfpropcb) {
+		WRITE_ONCE(fcs.stop, 1);
+		cur_ops->sync(); /* Wait for running callback to complete. */
+		cur_ops->cb_barrier(); /* Wait for queued callbacks. */
+		WARN_ON(READ_ONCE(fcs.stop) != 2);
+		destroy_rcu_head_on_stack(&fcs.rh);
+	}
 	/* Short runs might not contain a valid forward-progress attempt. */
 	WARN_ON(!tested && tested_tries >= 5);
 	torture_kthread_stopping("rcu_torture_fwd_prog");