Hello,
On Tue, Apr 15, 2014 at 10:44:37AM +0800, Li Zhong wrote:
> / *
> * This process might deadlock with another process trying to
> * remove this device:
> * This process holding the s_active of "online" attribute, and tries
> * to online/offline the device with some locks protecting hotplug.
> * Device removing process holding some locks protecting hotplug, and
> * tries to remove the "online" attribute, waiting for the s_active to
> * be released.
> *
> * The deadlock described above should be solved with
> * lock_device_hotplug_sysfs(). We temporarily drop the active
> * protection here to avoid some lockdep warnings.
> *
> * If device_hotplug_lock is forgotten to be used when removing
> * device(possibly some very simple device even don't need this lock?),
> * @dev could go away any time after dropping the active protection.
> * So increase its ref count before dropping active protection.
> * Though invoking device_{on|off}line() on a removed device seems
> * unreasonable, it should be less disastrous than playing with freed
> * @dev. Also, we might be able to have some mechanism abort
> * device_{on|off}line() if @dev already removed.
> */
Hmmm... I'm not sure I fully understand the problem. Does the code
ever try to remove "online" while holding cpu_add_remove_lock and,
when written 0, online knob grabs cpu_add_remove_lock? If so, that is
an actually possible deadlock, no?
Thanks.
--
tejun
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