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While testing memory hot-remove, I found following dead lock: Process #1141 is drmgr, trying to remove some memory, i.e. memory499. It holds the memory_hotplug_mutex, and blocks when trying to remove file "online" under dir memory499, in kernfs_drain(), at wait_event(root->deactivate_waitq, atomic_read(&kn->active) == KN_DEACTIVATED_BIAS); Process #1120 is trying to online memory499 by echo 1 > memory499/online In .kernfs_fop_write, it uses kernfs_get_active() to increase &kn->active, thus blocking process #1141. While itself is blocked later when trying to acquire memory_hotplug_mutex, which is held by process The backtrace of both processes are shown below: [<c000000001b18600>] 0xc000000001b18600 [<c000000000015044>] .__switch_to+0x144/0x200 [<c000000000263ca4>] .online_pages+0x74/0x7b0 [<c00000000055b40c>] .memory_subsys_online+0x9c/0x150 [<c00000000053cbe8>] .device_online+0xb8/0x120 [<c00000000053cd04>] .online_store+0xb4/0xc0 [<c000000000538ce4>] .dev_attr_store+0x64/0xa0 [<c00000000030f4ec>] .sysfs_kf_write+0x7c/0xb0 [<c00000000030e574>] .kernfs_fop_write+0x154/0x1e0 [<c000000000268450>] .vfs_write+0xe0/0x260 [<c000000000269144>] .SyS_write+0x64/0x110 [<c000000000009ffc>] syscall_exit+0x0/0x7c [<c000000001b18600>] 0xc000000001b18600 [<c000000000015044>] .__switch_to+0x144/0x200 [<c00000000030be14>] .__kernfs_remove+0x204/0x300 [<c00000000030d428>] .kernfs_remove_by_name_ns+0x68/0xf0 [<c00000000030fb38>] .sysfs_remove_file_ns+0x38/0x60 [<c000000000539354>] .device_remove_attrs+0x54/0xc0 [<c000000000539fd8>] .device_del+0x158/0x250 [<c00000000053a104>] .device_unregister+0x34/0xa0 [<c00000000055bc14>] .unregister_memory_section+0x164/0x170 [<c00000000024ee18>] .__remove_pages+0x108/0x4c0 [<c00000000004b590>] .arch_remove_memory+0x60/0xc0 [<c00000000026446c>] .remove_memory+0x8c/0xe0 [<c00000000007f9f4>] .pseries_remove_memblock+0xd4/0x160 [<c00000000007fcfc>] .pseries_memory_notifier+0x27c/0x290 [<c0000000008ae6cc>] .notifier_call_chain+0x8c/0x100 [<c0000000000d858c>] .__blocking_notifier_call_chain+0x6c/0xe0 [<c00000000071ddec>] .of_property_notify+0x7c/0xc0 [<c00000000071ed3c>] .of_update_property+0x3c/0x1b0 [<c0000000000756cc>] .ofdt_write+0x3dc/0x740 [<c0000000002f60fc>] .proc_reg_write+0xac/0x110 [<c000000000268450>] .vfs_write+0xe0/0x260 [<c000000000269144>] .SyS_write+0x64/0x110 [<c000000000009ffc>] syscall_exit+0x0/0x7c This patch uses lock_device_hotplug() to protect remove_memory() called in pseries_remove_memblock(), which is also stated before function remove_memory(): * NOTE: The caller must call lock_device_hotplug() to serialize hotplug * and online/offline operations before this call, as required by * try_offline_node(). */ void __ref remove_memory(int nid, u64 start, u64 size) With this lock held, the other process(#1120 above) trying to online the memory block will retry the system call when calling lock_device_hotplug_sysfs(), and finally find No such device error. Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org> |
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