driver-core/cpu: Expose hotpluggability to the rest of the kernel

When architectures register CPUs, they indicate whether the CPU allows
hotplugging; notably, x86 and ARM don't allow hotplugging CPU 0.
Userspace can easily query the hotpluggability of a CPU via sysfs;
however, the kernel has no convenient way of accessing that property in
an architecture-independent way.  While the kernel can simply try it and
see, some code needs to distinguish between "hotplug failed" and
"hotplug has no hope of working on this CPU"; for example, rcutorture's
CPU hotplug tests want to avoid drowning out real hotplug failures with
expected failures.

Expose this property via a new cpu_is_hotpluggable function, so that the
rest of the kernel can access it in an architecture-independent way.
Signed-off-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

drivers/base/cpu.c

@@ -247,6 +247,13 @@ struct sys_device *get_cpu_sysdev(unsigned cpu)
 }
 EXPORT_SYMBOL_GPL(get_cpu_sysdev);
 
+bool cpu_is_hotpluggable(unsigned cpu)
+{
+	struct sys_device *dev = get_cpu_sysdev(cpu);
+	return dev && container_of(dev, struct cpu, sysdev)->hotpluggable;
+}
+EXPORT_SYMBOL_GPL(cpu_is_hotpluggable);
+
 int __init cpu_dev_init(void)
 {
 	int err;

include/linux/cpu.h

@@ -27,6 +27,7 @@ struct cpu {
 
 extern int register_cpu(struct cpu *cpu, int num);
 extern struct sys_device *get_cpu_sysdev(unsigned cpu);
+extern bool cpu_is_hotpluggable(unsigned cpu);
 
 extern int cpu_add_sysdev_attr(struct sysdev_attribute *attr);
 extern void cpu_remove_sysdev_attr(struct sysdev_attribute *attr);