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?
---
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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