Re: memory hot-add: the kernel can notify udev daemon before creating the sys file state?

[Nathan Fontenot <nfont@linux.vnet.ibm.com>]

On 05/25/2014 10:41 AM, DX Cui wrote:
> On Fri, May 23, 2014 at 8:27 PM, DX Cui <rijcos@gmail.com> wrote:
>> Hi all,
>> I think I found out the root cause: when memory hotplug was introduced in 2005:
>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=3947be1969a9ce455ec30f60ef51efb10e4323d1
>> there was a race condition in:
>>
>> + static int add_memory_block(unsigned long node_id, struct
>> mem_section *section,
>> + unsigned long state, int phys_device)
>> +{
>> ...
>> + ret = register_memory(mem, section, NULL);
>> + if (!ret)
>> +        ret = mem_create_simple_file(mem, phys_index);
>> + if (!ret)
>> +        ret = mem_create_simple_file(mem, state);
>>
>> Here, first, add_memory_block() invokes register_memory() ->
>> sysdev_register() -> sysdev_add()->
>> kobject_uevent(&sysdev->kobj, KOBJ_ADD) to notify udev daemon, then
>> invokes mem_create_simple_file(). If the current execution is preempted
>> between the 2 steps, the issue I reported in the previous mail can happen.
>>
>> Luckily a commit in 2013 has fixed this issue undesignedly:
>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=96b2c0fc8e74a615888e2bedfe55b439aa4695e1
>>
>> It looks the new "register_memory() --> ... -> device_add()" path has the
>> correct order for sysfs creation and notification udev.
>>

Correct. that patch does fix this issue, though that was not the primary reason
for doing the patch. Always nice when a patch has unintended positive side affects.
 
>> It would be great if you can confirm my analysis. :-)
> 
> Any comments?
> I think we need to backport the patch
> 96b2c0fc8e74a615888e2bedfe55b439aa4695e1 to <=3.9 stable kernels.
> 

Although I have seen any issues because of this issue I agree that the fix
should be backported. Best to get rid of a known race condition before it
jumps up and bites us.

-Nathan

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