srcu: Avoid NULL dereference in srcu_torture_stats_print()

You really shouldn't invoke srcu_torture_stats_print() after invoking
cleanup_srcu_struct(), but there is really no reason to get a
compiler-obfuscated per-CPU-variable NULL pointer dereference as the
diagnostic.  This commit therefore checks for NULL ->sda and makes a
more polite console-message complaint in that case.
Co-developed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/srcutree.c

@@ -1477,9 +1477,14 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
 	idx = ssp->srcu_idx & 0x1;
 	if (ss_state < 0 || ss_state >= ARRAY_SIZE(srcu_size_state_name))
 		ss_state_idx = ARRAY_SIZE(srcu_size_state_name) - 1;
-	pr_alert("%s%s Tree SRCU g%ld state %d (%s) per-CPU(idx=%d):",
+	pr_alert("%s%s Tree SRCU g%ld state %d (%s)",
 		 tt, tf, rcu_seq_current(&ssp->srcu_gp_seq), ss_state,
-		 srcu_size_state_name[ss_state_idx], idx);
+		 srcu_size_state_name[ss_state_idx]);
+	if (!ssp->sda) {
+		// Called after cleanup_srcu_struct(), perhaps.
+		pr_cont(" No per-CPU srcu_data structures (->sda == NULL).\n");
+	} else {
+		pr_cont(" per-CPU(idx=%d):", idx);
 		for_each_possible_cpu(cpu) {
 			unsigned long l0, l1;
 			unsigned long u0, u1;
@@ -1508,6 +1513,7 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
 			s1 += c1;
 		}
 		pr_cont(" T(%ld,%ld)\n", s0, s1);
+	}
 	if (READ_ONCE(ssp->srcu_size_state) == SRCU_SIZE_SMALL && convert_to_big == 2)
 		WRITE_ONCE(ssp->srcu_size_state, SRCU_SIZE_ALLOC);
 }