Messages in this thread | | | Subject | Re: [RFC PATCH v3] Use kernfs_break_active_protection() for device online store callbacks | From | Li Zhong <> | Date | Wed, 16 Apr 2014 09:41:40 +0800 |
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On Tue, 2014-04-15 at 10:50 -0400, Tejun Heo wrote: > 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?
Yes.
In acpi_processor_remove(), cpu_maps_update_begin() is called to hold cpu_add_remove_lock, and then arch_unregister_cpu calls unregister_cpu(), which will try to remove dir cpu1 including "online".
while written 0 to online, cpu_down() will also try to grab cpu_add_remove_lock with cpu_maps_update_begin().
> If so, that is > an actually possible deadlock, no?
Yes, but it seems to me that it is solved in commit 5e33bc41, which uses lock_device_hotplug_sysfs() to return a restart syscall error if not able to try lock the device_hotplug_lock. That also requires the device removing code path to take the device_hotplug_lock.
Thanks, Zhong
> > Thanks. >
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