On Mon, Jul 09, 2012 at 01:13:04PM +0200, Michal Hocko wrote:
>On Fri 06-07-12 11:09:38, Gavin Shan wrote:
>> Apart from call to sparse_index_init() during boot stage, the function
>> is mainly used for hotplug case as follows and protected by hotplug
>
>mainly? Who are the others?
>
The function can possibilly be called during hotplug as well as boot stage
as follows. The others means "boot stage" here :-)
sparse_index_init
memory_present
sparse_memory_present_with_active_regions
>> mutex "mem_hotplug_mutex". So we needn't the spinlock in sparse_index_init().
>
>I think you are right but the changelog should be more convincing. It
>would be also good to mention the origin motivation for the lock (I
>couldn't find it in the history - Dave?).
>
Copy Dave's changelog for the original patch again.
---
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;
However, in the case where two copies of sparse_index_init() _do_ race,
which is probablly caused by making online for multiple memory sections
that depend on same entry of array mem_section[] simultaneously from
different CPUs.
---
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.
---
Thanks,
Gavin
>>
>> sparse_index_init
>> sparse_add_one_section
>> __add_section
>> __add_pages
>> arch_add_memory
>> add_memory
>>
>> Signed-off-by: Gavin Shan <shangw@xxxxxxxxxxxxxxxxxx>
>> ---
>> mm/sparse.c | 14 +-------------
>> 1 file changed, 1 insertion(+), 13 deletions(-)
>>
>> diff --git a/mm/sparse.c b/mm/sparse.c
>> index 8b8edfb..4437c6c 100644
>> --- a/mm/sparse.c
>> +++ b/mm/sparse.c
>> @@ -77,7 +77,6 @@ static struct mem_section noinline __init_refok *sparse_index_alloc(int nid)
>>
>> static int __meminit sparse_index_init(unsigned long section_nr, int nid)
>> {
>> - static DEFINE_SPINLOCK(index_init_lock);
>> unsigned long root = SECTION_NR_TO_ROOT(section_nr);
>> struct mem_section *section;
>> int ret = 0;
>> @@ -88,20 +87,9 @@ static int __meminit sparse_index_init(unsigned long section_nr, int nid)
>> section = sparse_index_alloc(nid);
>> if (!section)
>> return -ENOMEM;
>> - /*
>> - * This lock keeps two different sections from
>> - * reallocating for the same index
>> - */
>> - spin_lock(&index_init_lock);
>> -
>> - if (mem_section[root]) {
>> - ret = -EEXIST;
>> - goto out;
>> - }
>>
>> mem_section[root] = section;
>> -out:
>> - spin_unlock(&index_init_lock);
>> +
>> return ret;
>> }
>> #else /* !SPARSEMEM_EXTREME */
>> --
>> 1.7.9.5
>>
>> --
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>
>--
>Michal Hocko
>SUSE Labs
>SUSE LINUX s.r.o.
>Lihovarska 1060/12
>190 00 Praha 9
>Czech Republic
>
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>To unsubscribe, send a message with 'unsubscribe linux-mm' in
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>
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