Re: [PATCH v2 3/3] mm/sparse: remove index_init_lock

[Michal Hocko <mhocko@suse.cz>]

On Fri 13-07-12 10:01:22, Gavin Shan wrote:
> 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.
> 
> Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>

Reviewed-by: Michal Hocko <mhocko@suse.cz>

> ---
>  mm/sparse.c |   14 +-------------
>  1 files changed, 1 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 51950de..40b1100 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 */
> -- 
> 1.7.5.4
> 
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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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