Messages in this thread | | | Date | Tue, 15 Apr 2014 10:50:17 -0400 | From | Tejun Heo <> | Subject | Re: [RFC PATCH v3] Use kernfs_break_active_protection() for device online store callbacks |
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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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