Re: [linux-usb-devel] [PATCH] back out sysfs reference count change

[Maneesh Soni <maneesh@in.ibm.com>]

On Tue, Mar 30, 2004 at 06:19:15PM -0800, Andrew Morton wrote:
> 
> But it looks like that's all in a faraway perfect world, and Greg is going
> to fix stuff up somehow ;)

For convenience I will explain the race here..

cpu 0							cpu 1
kobject_unregister()				   sysfs_open_file()
  kobject_del()					     check_perm()
    sysfs_remove_dir()					   :
     (dentry remains alive due to ref. taken 		   :
      on the way to sysfs_open_file)			   :
  kobject_put()					   	   :
    kobject_cleanup()					kobject_get(->d_fsdata)

cpu 1 could end up referring to a freed kobject through dentry->d_fsdata or
starts spitting Badness in kobject_get at lib/kobject.c:429. For triggering 
this race try running these two loops simultaneously on SMP 

# while true; do insmod drivers/net/dummy.ko; rmmod dummy; done
# while true; do find /sys/class/net | xargs cat; done

Probably it can be solved by making sure that when sysfs file is 
opened/read/written some _race_ free check is done and fail if kobject if gone. 

Maneesh


-- 
Maneesh Soni
Linux Technology Center, 
IBM Software Lab, Bangalore, India
email: maneesh@in.ibm.com
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