rcu: Report error for bad rcu_nocbs= parameter values

This commit prints a console message when cpulist_parse() reports a
bad list of CPUs, and sets all CPUs' bits in that case.  The reason for
setting all CPUs' bits is that this is the safe(r) choice for real-time
workloads, which would normally be the ones using the rcu_nocbs= kernel
boot parameter.  Either way, later RCU console log messages list the
actual set of CPUs whose RCU callbacks will be offloaded.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree_plugin.h

@@ -1772,14 +1772,22 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp)
  */
 
 
-/* Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters. */
+/*
+ * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
+ * The string after the "rcu_nocbs=" is either "all" for all CPUs, or a
+ * comma-separated list of CPUs and/or CPU ranges.  If an invalid list is
+ * given, a warning is emitted and all CPUs are offloaded.
+ */
 static int __init rcu_nocb_setup(char *str)
 {
 	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
 	if (!strcasecmp(str, "all"))
 		cpumask_setall(rcu_nocb_mask);
 	else
-		cpulist_parse(str, rcu_nocb_mask);
+		if (cpulist_parse(str, rcu_nocb_mask)) {
+			pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
+			cpumask_setall(rcu_nocb_mask);
+		}
 	return 1;
 }
 __setup("rcu_nocbs=", rcu_nocb_setup);