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

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

Andrew Morton wrote:
> 
>> ---
>> 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);
> 
> It's strange to add a might_sleep() to a function which doesn't sleep.

It might sleep indeed. I prefer to add a might_sleep()/cond_sched()
for a sleepable function.

> 
> The patch adds no callers to this function.  This is significant
> because it would be quite interesting to find out which subsystem(s)
> you've found to have this deadlock.  I do think that we should look at
> alternative (non-trylocky) ways of fixing them.
> 

This problem exist in cgroup/cpuset. and Max Krasnyansky fix it:
(non-trylocky way)
 *
 * The rebuild_sched_domains() and partition_sched_domains()
 * routines must nest cgroup_lock() inside get_online_cpus(),
 * but such cpuset changes as these must nest that locking the
 * other way, holding cgroup_lock() for much of the code.
 *
 * So in order to avoid an ABBA deadlock, the cpuset code handling
 * these user changes delegates the actual sched domain rebuilding
 * to a separate workqueue thread, which ends up processing the
 * above do_rebuild_sched_domains() function.
 */
static void async_rebuild_sched_domains(void)
{
	queue_work(cpuset_wq, &rebuild_sched_domains_work);
}


get_online_cpus() is so a coarsely granular lock, and
try_get_online_cpus() fails rarely. The kernel indeed needs
try_get_online_cpus().

Paul will use it: 
http://lkml.org/lkml/2009/5/22/332

Lai