Re: [PATCH] race condition fixing in sysfs_create_dir

[Dennis Chen <xschen@tnsoft.com.cn>]

On 07/26/2013 09:38 PM, Tejun Heo wrote:
> Hello,
>
> On Fri, Jul 26, 2013 at 05:59:00PM +0800, Dennis Chen wrote:
>> On 07/26/2013 05:49 PM, Dennis Chen wrote:
>>
>>> The patch is trying its best to avoid creating a dir under a parent dir which is removing from
>>> the system:
>>>      PATH0 (create a dir under 'PARENT/...')         PATH1 (remove the 'PARENT/...')
>>>           sysfs_create_dir() {                         sysfs_remove_dir() {
>>>           ...                                          ...
>>>           if (kobj->parent)                            spin_lock(&sysfs_assoc_lock);
>>>          parent_sd = kobj->parent->sd;  <----- kobj->sd = NULL;
>>>       else                                         spin_unlock(&sysfs_assoc_lock);
>>>          parent_sd = &sysfs_root;
>>> Suppose PATH1 enter the critical section first, then PATH0 begin to execute before kobj->sd
>>> has been reset to NULL, possibly PATH0 will get a non-NULL parent_sd since lack of the
>>> sysfs_assoc_lock protection in PATH0. In this case, PATH0 think it has a valid parent_sd which
>>> can be freed by PATH1 in the followed, refer to the comments in the patch. Maybe we need
>>> to figure out a perfect solution to solve the race condition, although the codes in question are
>>> in slow path...
> I don't think sysfs is supposed to handle multiple actors trying to
> populate and destroy the directory at the same time at all, so this
> seems kinda moot.  Do you have a case where this actually matters?
>
> Thanks.
>
hello,Tejun. Nice. But seems I still have different opinion :). If you look at the 'sysfs_do_create_link_sd()'
code, you will find a comment "target->sd can go away beneath us but is protected with sysfs_assoc_lock.
Fetch target_sd from it", don't you think the sysfs_create_dir is the same as the sysfs_do_create_link_sd()
essentially? if the answer is yes meaning the parent dir can go away when its sub-dir is creating by sysfs_create_dir,
then the similar action should be taken as sysfs_create_link does. right?