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    SubjectRe: [linux-usb-devel] [PATCH] back out sysfs reference count change
    On Wed, 31 Mar 2004, Maneesh Soni wrote:

    > 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

    Here's a suggestion. At the start of check_perm() grab the dentry
    semaphore, then check whether d_fsdata is NULL, if it isn't then do the
    kobject_get(), then unlock the semaphore.

    Alan Stern

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