[PATCH 2/2] cpuhotplug: introduce try_get_online_cpus()

[Lai Jiangshan <laijs@cn.fujitsu.com>]

get_online_cpus() is a typically coarsely granular lock.
It's a source of ABBA deadlock.

Thanks to the CPU notifiers, Some subsystem's global lock will
be required after cpu_hotplug.rwlock. Subsystem's global lock
is coarsely granular lock too, thus a lot's of lock in kernel
should be required after cpu_hotplug.rwlock(if we need
cpu_hotplug.rwlock held too)

Otherwise it may come to a ABBA deadlock like this:

thread 1                                      |        thread 2
_cpu_down()                                   |  Lock a-kernel-lock.
  cpu_hotplug_begin()                         |
    down_write(&cpu_hotplug.rwlock)           |
  __raw_notifier_call_chain(CPU_DOWN_PREPARE) |  get_online_cpus()
------------------------------------------------------------------------
    Lock a-kernel-lock.(wait thread2)         |    down_read(&cpu_hotplug.rwlock)
                                                   (wait thread 1)

But CPU online/offline are happened very rarely, get_online_cpus()
returns success quickly in all probability.
So it's an asinine behavior that get_online_cpus() is not allowed
to be required after we had held "a-kernel-lock".

To dispel the ABBA deadlock, this patch introduces
try_get_online_cpus(). It returns fail very rarely. It gives the
caller a chance to select an alternative way to finish works,
instead of sleeping or deadlock.

Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 2643d84..98f5c4b 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -104,6 +104,7 @@ extern struct sysdev_class cpu_sysdev_class;
 
 extern void get_online_cpus(void);
 extern void put_online_cpus(void);
+extern int try_get_online_cpus(void);
 #define hotcpu_notifier(fn, pri) {				\
 	static struct notifier_block fn##_nb __cpuinitdata =	\
 		{ .notifier_call = fn, .priority = pri };	\
@@ -117,6 +118,7 @@ int cpu_down(unsigned int cpu);
 
 #define get_online_cpus()	do { } while (0)
 #define put_online_cpus()	do { } while (0)
+#define try_get_online_cpus()	(1)
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 62198ec..e948f19 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -66,6 +66,15 @@ void put_online_cpus(void)
 }
 EXPORT_SYMBOL_GPL(put_online_cpus);
 
+int try_get_online_cpus(void)
+{
+	might_sleep();
+	if (cpu_hotplug.active_writer == current)
+		return 1;
+	return down_read_trylock(&cpu_hotplug.rwlock);
+
+}
+EXPORT_SYMBOL_GPL(try_get_online_cpus);
 #endif	/* CONFIG_HOTPLUG_CPU */
 
 /*