Messages in this thread | | | Date | Thu, 10 Apr 2014 09:31:16 -0400 | From | Tejun Heo <> | Subject | Re: [RFC PATCH] Suppress a device hot remove related lockdep warning |
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Hello,
On Thu, Apr 10, 2014 at 05:18:34PM +0800, Li Zhong wrote: > I noticed following lockdep warning when trying acpi hot-remove cpus: > > [84154.204080] ====================================================== > [84154.204080] [ INFO: possible circular locking dependency detected ] > [84154.204080] 3.14.0-next-20140408+ #24 Tainted: G W > [84154.204080] ------------------------------------------------------- > [84154.204080] bash/777 is trying to acquire lock: > [84154.204080] (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff810664a7>] cpu_maps_update_begin+0x17/0x20 > [84154.213203] > [84154.213203] but task is already holding lock: > [84154.213203] (s_active#79){++++.+}, at: [<ffffffff81256e14>] kernfs_fop_write+0xe4/0x190 > [84154.213203] > [84154.213203] which lock already depends on the new lock. > [84154.213203] > [84154.213203] > [84154.213203] the existing dependency chain (in reverse order) is: > [84154.213203] ... > [84154.213203] Possible unsafe locking scenario: > [84154.213203] > [84154.213203] CPU0 CPU1 > [84154.213203] ---- ---- > [84154.213203] lock(s_active#79); > [84154.213203] lock(cpu_hotplug.lock#2); > [84154.213203] lock(s_active#79); > [84154.213203] lock(cpu_add_remove_lock); > [84154.213203] > [84154.213203] *** DEADLOCK *** > ............. > > The deadlock itself seems already fixed in commit 5e33bc41. > > This patch uses DEVICE_ATTR_IGNORE_LOCKDEP for "online" attr to suppress > this lockdep warning. But I'm a little afraid it might also hide > (future) potential real dead lock scenarios?
I suppose this is happening because offlining a cpu involves removing some sysfs files? If the file isn't trying to remove itself, it should be okay but I think it could be better to use kernfs_break_active_protection() from online callback so that the operation is actually outside the dependency chain rather than just suppressing lockdep.
Thanks.
-- tejun
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