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SubjectLockdep problem involving sysfs_mutex in ext4 in linux-next
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I'm occasionally seeing a circular locking dependency in linux-next when
I unmount an ext4 filesystem, apparently involving sysfs_mutex in
sysfs_addrm_start(), and I have no idea what's going on. Some help from
kobject experts would be greatly appreciated. I assume I must be doing
something wrong. I took the basic pattern from btrfs, but I suspect I
may have broken some of the kobject lifetime rules when I adapted what I
needed for ext4.

It doesn't happen all the time, and it seems to be caused by how I'm
releasing the ext4's sysfs kobject. In ext4_put_super():

kobject_put(&sbi->s_kobj);
wait_for_completion(&sbi->s_kobj_unregister);

The circular locking dependency is:

[32667.071746] =======================================================
[32667.071751] [ INFO: possible circular locking dependency detected ]
[32667.071755] 2.6.29-rc7-00043-gafbc462 #11
[32667.071758] -------------------------------------------------------
[32667.071762] umount/2310 is trying to acquire lock:
[32667.071764] (sysfs_mutex){--..}, at: [<c01f7130>] sysfs_addrm_start+0x28/0x9a
[32667.071778]
[32667.071779] but task is already holding lock:
[32667.071782] (&type->s_lock_key#5){--..}, at: [<c01b752b>] lock_super+0x26/0x28

And the stack trace is:

[32667.072279] [<c01f7130>] ? sysfs_addrm_start+0x28/0x9a
[32667.072284] [<c04c60ca>] __mutex_lock_common+0xdc/0x33a
[32667.072289] [<c01f7130>] ? sysfs_addrm_start+0x28/0x9a
[32667.072294] [<c04c63cf>] mutex_lock_nested+0x33/0x3b
[32667.072300] [<c01f7130>] ? sysfs_addrm_start+0x28/0x9a
[32667.072304] [<c01f7130>] sysfs_addrm_start+0x28/0x9a
[32667.072309] [<c01f75ba>] sysfs_remove_dir+0x77/0xab
[32667.072314] [<c02d5b2f>] kobject_del+0xf/0x2c
[32667.072319] [<c02d5c80>] kobject_release+0x134/0x1cb
[32667.072323] [<c02d5b4c>] ? kobject_release+0x0/0x1cb
[32667.072328] [<c02d69e4>] kref_put+0x3c/0x4a
[32667.072333] [<c02d5ac8>] kobject_put+0x37/0x3c
[32667.072337] [<c021cd89>] ext4_put_super+0xab/0x215
[32667.072342] [<c01b7fed>] generic_shutdown_super+0x62/0xe3
[32667.072346] [<c01b8090>] kill_block_super+0x22/0x36
[32667.072352] [<c01e6f52>] ? vfs_quota_off+0x0/0x17
[32667.072357] [<c01b815b>] deactivate_super+0x5c/0x6f
[32667.072361] [<c01c8c34>] mntput_no_expire+0xd4/0x106
[32667.072366] [<c01c9138>] sys_umount+0x2a1/0x2c6
[32667.072370] [<c01c916f>] sys_oldumount+0x12/0x14
[32667.072375] [<c0117ff6>] syscall_call+0x7/0xb
[32667.072380] [<c0110000>] ? x86_decode_insn+0x349/0xbc9

Can anybody tell me what I did wrong? Thanks!!

- Ted


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