rcu: Once again use NMI-based stack traces in stall warnings

This commit is for all intents and purposes a revert of bc1dce51
("rcu: Don't use NMIs to dump other CPUs' stacks").  The reason to suppose
that this can now safely be reverted is the presence of 42a0bb3f
("printk/nmi: generic solution for safe printk in NMI"), which is said
to have made NMI-based stack dumps safe.

However, this reversion keeps one nice property of bc1dce51
("rcu: Don't use NMIs to dump other CPUs' stacks"), namely that
only those CPUs blocking the grace period are dumped.  The new
trigger_single_cpu_backtrace() is used to make this happen, as
suggested by Josh Poimboeuf.
Reported-by: NVince Weaver <vincent.weaver@maine.edu>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: NPetr Mladek <pmladek@suse.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

kernel/rcu/tree.c

@@ -1278,7 +1278,10 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
 }
 
 /*
- * Dump stacks of all tasks running on stalled CPUs.
+ * Dump stacks of all tasks running on stalled CPUs.  First try using
+ * NMIs, but fall back to manual remote stack tracing on architectures
+ * that don't support NMI-based stack dumps.  The NMI-triggered stack
+ * traces are more accurate because they are printed by the target CPU.
  */
 static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
 {
@@ -1288,11 +1291,10 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
 
 	rcu_for_each_leaf_node(rsp, rnp) {
 		raw_spin_lock_irqsave_rcu_node(rnp, flags);
-		if (rnp->qsmask != 0) {
-			for_each_leaf_node_possible_cpu(rnp, cpu)
-				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
+		for_each_leaf_node_possible_cpu(rnp, cpu)
+			if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu))
+				if (!trigger_single_cpu_backtrace(cpu))
 					dump_cpu_task(cpu);
-		}
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}
 }