ACPI / processor: Acquire writer lock to update CPU maps

CPU system maps are protected with reader/writer locks.  The reader
lock, get_online_cpus(), assures that the maps are not updated while
holding the lock.  The writer lock, cpu_hotplug_begin(), is used to
udpate the cpu maps along with cpu_maps_update_begin().

However, the ACPI processor handler updates the cpu maps without
holding the the writer lock.

acpi_map_lsapic() is called from acpi_processor_hotadd_init() to
update cpu_possible_mask and cpu_present_mask.  acpi_unmap_lsapic()
is called from acpi_processor_remove() to update cpu_possible_mask.
Currently, they are either unprotected or protected with the reader
lock, which is not correct.

For example, the get_online_cpus() below is supposed to assure that
cpu_possible_mask is not changed while the code is iterating with
for_each_possible_cpu().

        get_online_cpus();
        for_each_possible_cpu(cpu) {
		:
        }
        put_online_cpus();

However, this lock has no protection with CPU hotplug since the ACPI
processor handler does not use the writer lock when it updates
cpu_possible_mask.  The reader lock does not serialize within the
readers.

This patch protects them with the writer lock with cpu_hotplug_begin()
along with cpu_maps_update_begin(), which must be held before calling
cpu_hotplug_begin().  It also protects arch_register_cpu() /
arch_unregister_cpu(), which creates / deletes a sysfs cpu device
interface.  For this purpose it changes cpu_hotplug_begin() and
cpu_hotplug_done() to global and exports them in cpu.h.

Signed-off-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpi_processor.c

@@ -178,14 +178,17 @@ static int acpi_processor_hotadd_init(struct acpi_processor *pr)
 	if (ACPI_FAILURE(status) || !(sta & ACPI_STA_DEVICE_PRESENT))
 		return -ENODEV;
 
+	cpu_maps_update_begin();
+	cpu_hotplug_begin();
+
 	ret = acpi_map_lsapic(pr->handle, &pr->id);
 	if (ret)
-		return ret;
+		goto out;
 
 	ret = arch_register_cpu(pr->id);
 	if (ret) {
 		acpi_unmap_lsapic(pr->id);
-		return ret;
+		goto out;
 	}
 
 	/*
@@ -195,7 +198,11 @@ static int acpi_processor_hotadd_init(struct acpi_processor *pr)
 	 */
 	pr_info("CPU%d has been hot-added\n", pr->id);
 	pr->flags.need_hotplug_init = 1;
-	return 0;
+
+out:
+	cpu_hotplug_done();
+	cpu_maps_update_done();
+	return ret;
 }
 #else
 static inline int acpi_processor_hotadd_init(struct acpi_processor *pr)
@@ -452,11 +459,15 @@ static void acpi_processor_remove(struct acpi_device *device)
 	per_cpu(processor_device_array, pr->id) = NULL;
 	per_cpu(processors, pr->id) = NULL;
 
+	cpu_maps_update_begin();
+	cpu_hotplug_begin();
+
 	/* Remove the CPU. */
-	get_online_cpus();
 	arch_unregister_cpu(pr->id);
 	acpi_unmap_lsapic(pr->id);
-	put_online_cpus();
+
+	cpu_hotplug_done();
+	cpu_maps_update_done();
 
 	try_offline_node(cpu_to_node(pr->id));
 

include/linux/cpu.h

@@ -172,6 +172,8 @@ extern struct bus_type cpu_subsys;
 #ifdef CONFIG_HOTPLUG_CPU
 /* Stop CPUs going up and down. */
 
+extern void cpu_hotplug_begin(void);
+extern void cpu_hotplug_done(void);
 extern void get_online_cpus(void);
 extern void put_online_cpus(void);
 extern void cpu_hotplug_disable(void);
@@ -197,6 +199,8 @@ static inline void cpu_hotplug_driver_unlock(void)
 
 #else		/* CONFIG_HOTPLUG_CPU */
 
+static inline void cpu_hotplug_begin(void) {}
+static inline void cpu_hotplug_done(void) {}
 #define get_online_cpus()	do { } while (0)
 #define put_online_cpus()	do { } while (0)
 #define cpu_hotplug_disable()	do { } while (0)

kernel/cpu.c

@@ -113,7 +113,7 @@ EXPORT_SYMBOL_GPL(put_online_cpus);
  * get_online_cpus() not an api which is called all that often.
  *
  */
-static void cpu_hotplug_begin(void)
+void cpu_hotplug_begin(void)
 {
 	cpu_hotplug.active_writer = current;
 
@@ -127,7 +127,7 @@ static void cpu_hotplug_begin(void)
 	}
 }
 
-static void cpu_hotplug_done(void)
+void cpu_hotplug_done(void)
 {
 	cpu_hotplug.active_writer = NULL;
 	mutex_unlock(&cpu_hotplug.lock);
@@ -154,10 +154,7 @@ void cpu_hotplug_enable(void)
 	cpu_maps_update_done();
 }
 
-#else /* #if CONFIG_HOTPLUG_CPU */
-static void cpu_hotplug_begin(void) {}
-static void cpu_hotplug_done(void) {}
-#endif	/* #else #if CONFIG_HOTPLUG_CPU */
+#endif	/* CONFIG_HOTPLUG_CPU */
 
 /* Need to know about CPUs going up/down? */
 int __ref register_cpu_notifier(struct notifier_block *nb)