Re: [PATCH 1/2] cpuhotplug: use rw_semaphore for cpu_hotplug

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

Oleg Nesterov wrote:
> On 05/29, Andrew Morton wrote:
>> On Fri, 29 May 2009 16:29:30 +0800
>> Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
>>
>>> Current get_online_cpus()/put_online_cpus() re-implement
>>> a rw_semaphore,
>>> so it is converted to a real rw_semaphore in this fix.
>>> It simplifies codes, and is good for read.
>>>  static struct {
>>> -	struct task_struct *active_writer;
>>> -	struct mutex lock; /* Synchronizes accesses to refcount, */
>>>  	/*
>>> -	 * Also blocks the new readers during
>>> -	 * an ongoing cpu hotplug operation.
>>> +	 * active_writer makes get_online_cpus()/put_online_cpus() are allowd
>>> +	 * to be nested in cpu_hotplug_begin()/cpu_hotplug_done().
>>> +	 *
>>> +	 * Thus, get_online_cpus()/put_online_cpus() can be called in
>>> +	 * CPU notifiers.
>>>  	 */
>>> -	int refcount;
>>> +	struct task_struct *active_writer;
>>> +	struct rw_semaphore rwlock;
>>>  } cpu_hotplug;
> 
> But, afaics, down_write() blocks new readers.
> 
> This means that with this patch get_online_cpus() is not recursive, no?
> 

down_read()/up_read() can be nested within down_read()/up_read(),
so get_online_cpus() is recursive.

And thanks to cpu_hotplug.active_writer, get_online_cpus()/put_online_cpus()
are allowd to be nested in cpu_hotplug_begin()/cpu_hotplug_done().
So cpu_hotplug_begin() DO NOT blocks readers who are in CPU notifiers.