rcutorture: Add test of holding scheduler locks across rcu_read_unlock()

Now that it should be safe to hold scheduler locks across
rcu_read_unlock(), even in cases where the corresponding RCU read-side
critical section might have been preempted and boosted, the commit adds
a test of this capability to rcutorture.  This has been tested on current
mainline (which can deadlock in this situation), and lockdep duly reported
the expected deadlock.  On -rcu, lockdep is silent, thus far, anyway.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcutorture.c

@@ -1147,6 +1147,7 @@ static void rcutorture_one_extend(int *readstate, int newstate,
 				  struct torture_random_state *trsp,
 				  struct rt_read_seg *rtrsp)
 {
+	unsigned long flags;
 	int idxnew = -1;
 	int idxold = *readstate;
 	int statesnew = ~*readstate & newstate;
@@ -1181,8 +1182,15 @@ static void rcutorture_one_extend(int *readstate, int newstate,
 		rcu_read_unlock_bh();
 	if (statesold & RCUTORTURE_RDR_SCHED)
 		rcu_read_unlock_sched();
-	if (statesold & RCUTORTURE_RDR_RCU)
+	if (statesold & RCUTORTURE_RDR_RCU) {
+		bool lockit = !statesnew && !(torture_random(trsp) & 0xffff);
+
+		if (lockit)
+			raw_spin_lock_irqsave(&current->pi_lock, flags);
 		cur_ops->readunlock(idxold >> RCUTORTURE_RDR_SHIFT);
+		if (lockit)
+			raw_spin_unlock_irqrestore(&current->pi_lock, flags);
+	}
 
 	/* Delay if neither beginning nor end and there was a change. */
 	if ((statesnew || statesold) && *readstate && newstate)