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

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 */

/*






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