On Mon, Jul 02, 2012 at 05:46:28PM +0200, Michal Hocko wrote:
>On Mon 02-07-12 21:40:53, Gavin Shan wrote:
>> On Mon, Jul 02, 2012 at 11:43:31AM +0200, Michal Hocko wrote:
>> >On Mon 02-07-12 17:28:56, Gavin Shan wrote:
>> >> 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,
>> >> the one that loses the race will leak the "section" that
>> >> sparse_index_alloc() allocated for it. This patch fixes that leak.
>> >
>> >I would still like to hear how we can possibly race in this code path.
>> >I've thought that memory onlining is done from a single CPU.
>> >
>>
>> Hi Michael, how about to use 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;
>>
>> 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.
>
>And you really think that this clarified the things? You have just
>tweaked the comment to sound more obscure.
>
>OK, so you have pushed me into the code...
>If you had looked into the hotplug callchain up to add_memory you would
>have seen that the whole arch_add_memory -> __add_pages -> ... ->
>sparse_index_init is called with lock_memory_hotplug held so the hotplug
>cannot run from the multiple CPUs.
>
>I do not see any other users apart from boot time
>sparse_memory_present_with_active_regions and add_memory so I think that
>the lock is just a heritage from old days.
>
I just had quick go-through on the source code as you suggested and I
think you're right, Michal. So please drop this :-)
With CONFIG_ARCH_MEMORY_PROBE enabled on Power machines, following
functions would be included in hotplug path.
memory_probe_store
add_memory
lock_memory_hotplug /* protect the whole hotplug path */
arch_add_memory
__add_pages
__add_section
sparse_add_one_section
sparse_index_init
sparse_index_alloc
The mutex "mem_hotplug_mutex" will be hold by lock_memory_hotplug() to protect
the whole hotplug path. However, I'm wandering if we can remove the "index_init_lock"
of function sparse_index_init() since that sounds duplicate lock.
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;
if (mem_section[root])
return -EEXIST;
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);
if (ret)
sparse_index_free(section);
return ret;
}
Thanks,
Gavin
>So please make sure you are fixing a real issue rather than add another
>code which simply never gets executed.
>
>And no obscuring the changelog doesn't help anybody.
>
>> The one that loses the race will leak the "section" that
>> sparse_index_alloc() allocated for it. This patch fixes that leak.
>
>>
>> -----
>>
>> Thanks,
>> Gavin
>>
>> >>
>> >> Signed-off-by: Gavin Shan <shangw@xxxxxxxxxxxxxxxxxx>
>> >> ---
>> >> mm/sparse.c | 17 +++++++++++++++++
>> >> 1 file changed, 17 insertions(+)
>> >>
>> >> diff --git a/mm/sparse.c b/mm/sparse.c
>> >> index 781fa04..a6984d9 100644
>> >> --- a/mm/sparse.c
>> >> +++ b/mm/sparse.c
>> >> @@ -75,6 +75,20 @@ static struct mem_section noinline __init_refok *sparse_index_alloc(int nid)
>> >> return section;
>> >> }
>> >>
>> >> +static inline void __meminit sparse_index_free(struct mem_section *section)
>> >> +{
>> >> + unsigned long size = SECTIONS_PER_ROOT *
>> >> + sizeof(struct mem_section);
>> >> +
>> >> + if (!section)
>> >> + return;
>> >> +
>> >> + if (slab_is_available())
>> >> + kfree(section);
>> >> + else
>> >> + free_bootmem(virt_to_phys(section), size);
>> >> +}
>> >> +
>> >> static int __meminit sparse_index_init(unsigned long section_nr, int nid)
>> >> {
>> >> static DEFINE_SPINLOCK(index_init_lock);
>> >> @@ -102,6 +116,9 @@ static int __meminit sparse_index_init(unsigned long section_nr, int nid)
>> >> mem_section[root] = section;
>> >> out:
>> >> spin_unlock(&index_init_lock);
>> >> + if (ret)
>> >> + sparse_index_free(section);
>> >> +
>> >> return ret;
>> >> }
>> >> #else /* !SPARSEMEM_EXTREME */
>> >> --
>> >> 1.7.9.5
>> >>
>> >> --
>> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> >> the body to majordomo@xxxxxxxxx. For more info on Linux MM,
>> >> see: http://www.linux-mm.org/ .
>> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
>> >
>> >--
>> >Michal Hocko
>> >SUSE Labs
>> >SUSE LINUX s.r.o.
>> >Lihovarska 1060/12
>> >190 00 Praha 9
>> >Czech Republic
>> >
>> >--
>> >To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> >the body to majordomo@xxxxxxxxx. For more info on Linux MM,
>> >see: http://www.linux-mm.org/ .
>> >Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
>> >
>>
>> --
>> To unsubscribe, send a message with 'unsubscribe linux-mm' in
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>> see: http://www.linux-mm.org/ .
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>
>--
>Michal Hocko
>SUSE Labs
>SUSE LINUX s.r.o.
>Lihovarska 1060/12
>190 00 Praha 9
>Czech Republic
>
>--
>To unsubscribe, send a message with 'unsubscribe linux-mm' in
>the body to majordomo@xxxxxxxxx. For more info on Linux MM,
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>
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