From: Michal Hocko <mhocko@suse.cz>
To: Gavin Shan <shangw@linux.vnet.ibm.com>
Cc: linux-mm@kvack.org, dave@linux.vnet.ibm.com, rientjes@google.com,
akpm@linux-foundation.org
Subject: Re: [PATCH 3/3] mm/sparse: remove index_init_lock
Date: Tue, 10 Jul 2012 17:59:41 +0200 [thread overview]
Message-ID: <20120710155940.GG19223@tiehlicka.suse.cz> (raw)
In-Reply-To: <20120709115935.GA19355@shangw>
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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next prev parent reply other threads:[~2012-07-10 15:59 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-06 3:09 [PATCH v4 1/3] mm/sparse: optimize sparse_index_alloc Gavin Shan
2012-07-06 3:09 ` [PATCH v4 2/3] mm/sparse: more check on mem_section number Gavin Shan
2012-07-06 3:09 ` [PATCH 3/3] mm/sparse: remove index_init_lock Gavin Shan
2012-07-09 11:13 ` Michal Hocko
2012-07-09 11:59 ` Gavin Shan
2012-07-10 15:59 ` Michal Hocko [this message]
2012-07-12 5:48 ` Gavin Shan
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