Re: [PATCH 3/3] mm/sparse: remove index_init_lock

[Gavin Shan <shangw@linux.vnet.ibm.com>]

On Tue, Jul 10, 2012 at 05:59:41PM +0200, Michal Hocko wrote:
>On Mon 09-07-12 19:59:35, Gavin Shan wrote:
>[...]
>> Michal, How about the following changelog?
>> 
>> ---
>> 
>> sparse_index_init() is designed to be safe if two copies of it race.  It
>> uses "index_init_lock" to ensure that, even in the case of a race, only
>> one CPU will manage to do:
>> 
>> mem_section[root] = section;
>> 
>> On the other hand, sparse_index_init() is possiblly called during system
>> boot stage and hotplug path as follows. We need't lock during system boot
>> stage to protect "mem_section[root]" and the function has been protected by
>> hotplug mutex "mem_hotplug_mutex" as well in hotplug case. So we needn't the
>> spinklock in the function.
>
>The changelog is still hard to read but it's getting there slowly ;)
>What about the following?

Thanks, Michal. I will resend the patch with your changelog :-)

Thanks for your time.

Gavin

>---
>sparse_index_init uses index_init_lock spinlock to protect root
>mem_section assignment. The lock is not necessary anymore because the
>function is called only during the boot (during paging init which
>is executed only from a single CPU) and from the hotplug code (by
>add_memory via arch_add_memory) which uses mem_hotplug_mutex.
>
>The lock has been introduced by 28ae55c9 (sparsemem extreme: hotplug
>preparation) and sparse_index_init was used only during boot at that
>time. 
>Later when the hotplug code (and add_memory) was introduced there was
>no synchronization so it was possible to online more sections from
>the same root probably (though I am not 100% sure about that).
>The first synchronization has been added by 6ad696d2 (mm: allow memory
>hotplug and hibernation in the same kernel) which has been later
>replaced by the mem_hotplug_mutex - 20d6c96b (mem-hotplug: introduce
>{un}lock_memory_hotplug()).
>
>Let's remove the lock as it is not needed and it makes the code more
>confusing.
>---
>
>-- 
>Michal Hocko
>SUSE Labs
>SUSE LINUX s.r.o.
>Lihovarska 1060/12
>190 00 Praha 9    
>Czech Republic
>

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