Re: [PATCH] race condition fixing in sysfs_create_dir

From: Dennis Chen
Date: Tue Jul 30 2013 - 02:32:21 EST

On 07/26/2013 09:38 PM, Tejun Heo wrote:

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?


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?

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