rcu: Detect stalls caused by failure to propagate up rcu_node tree

If all CPUs have passed through quiescent states, then stalls might be
due to starvation of the grace-period kthread or to failure to propagate
the quiescent states up the rcu_node combining tree. The current stall
warning messages do not differentiate, so this commit adds a printout
of the root rcu_node structure's ->qsmask field.

Signed-off-by: Paul E. McKenney <[e-mail]>

kernel/rcu/tree.c

···
 		} else {
 			j = jiffies;
 			gpa = ACCESS_ONCE(rsp->gp_activity);
-			pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld\n",
+			pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
 			       rsp->name, j - gpa, j, gpa,
-			       jiffies_till_next_fqs);
+			       jiffies_till_next_fqs,
+			       rcu_get_root(rsp)->qsmask);
 			/* In this case, the current CPU might be at fault. */
 			sched_show_task(current);
 		}