Re: Lockdep problem involving sysfs_mutex in ext4 in linux-next

From: Theodore Tso
Date: Fri Mar 20 2009 - 18:25:39 EST

On Sun, Mar 15, 2009 at 12:05:08AM +0100, Peter Zijlstra wrote:
> On Sat, 2009-03-14 at 10:02 -0400, Theodore Ts'o wrote:
> > 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);
>
> Right that would require sysfs_mutex() while you're holding s_lock.

Really? I can't see anywhere else in the kernel where code outside of
fs/sysfs which grabs sysfs_mutex?

> And here you snipped the interesting bit here it tells us how you
> normally have the reverse lock order, namely:
>
> sysfs_mutex.
> s_lock

Ok, here's another such report:

[42993.140759] the existing dependency chain (in reverse order) is:
[42993.140762]
[42993.140762] -> #4 (&type->s_lock_key#6){--..}:
[42993.140769] [<c0162845>] __lock_acquire+0x9a6/0xb19
[42993.140776] [<c0162a13>] lock_acquire+0x5b/0x81
[42993.140780] [<c04c402a>] __mutex_lock_common+0xdc/0x33a
[42993.140787] [<c04c432f>] mutex_lock_nested+0x33/0x3b
[42993.140791] [<c01b7493>] lock_super+0x26/0x28
[42993.140795] [<c02173c1>] ext4_orphan_add+0x1ab/0x1ca
[42993.140801] [<c020fc39>] ext4_setattr+0x19b/0x2de
[42993.140805] [<c01c69d9>] notify_change+0x164/0x2af
[42993.140816] [<c01b54f3>] do_truncate+0x6b/0x84
[42993.140822] [<c01bea2b>] may_open+0x196/0x19c
[42993.140826] [<c01bf000>] do_filp_open+0x341/0x680
[42993.140830] [<c01b4906>] do_sys_open+0x47/0xbc
[42993.140834] [<c01b49c7>] sys_open+0x23/0x2b
[42993.140839] [<c0117ff6>] syscall_call+0x7/0xb
[42993.140844] [<ffffffff>] 0xffffffff
[42993.140852]
[42993.140853] -> #3 (jbd2_handle){--..}:
[42993.140858] [<c0162845>] __lock_acquire+0x9a6/0xb19
[42993.140862] [<c0162a13>] lock_acquire+0x5b/0x81
[42993.140866] [<c0235130>] jbd2_journal_start+0xea/0xf7
[42993.140871] [<c021ce94>] ext4_journal_start_sb+0x49/0x69
[42993.140876] [<c020fc20>] ext4_setattr+0x182/0x2de
[42993.140880] [<c01c69d9>] notify_change+0x164/0x2af
[42993.140884] [<c01b54f3>] do_truncate+0x6b/0x84
[42993.140888] [<c01bea2b>] may_open+0x196/0x19c
[42993.140892] [<c01bf000>] do_filp_open+0x341/0x680
[42993.140896] [<c01b4906>] do_sys_open+0x47/0xbc
[42993.140900] [<c01b49c7>] sys_open+0x23/0x2b
[42993.140905] [<c0117ff6>] syscall_call+0x7/0xb
[42993.140909] [<ffffffff>] 0xffffffff
[42993.140915]
[42993.140915] -> #2 (&sb->s_type->i_alloc_sem_key#4){----}:
[42993.140921] [<c0162845>] __lock_acquire+0x9a6/0xb19
[42993.140925] [<c0162a13>] lock_acquire+0x5b/0x81
[42993.140929] [<c04c4621>] down_read+0x37/0x74
[42993.140933] [<c020e42a>] ext4_page_mkwrite+0x36/0x16a
[42993.140937] [<c019ff30>] do_wp_page+0x1ad/0x60a
[42993.140942] [<c01a1a80>] handle_mm_fault+0x676/0x728
[42993.140946] [<c04c7702>] do_page_fault+0x333/0x7ce
[42993.140950] [<c04c5b57>] error_code+0x77/0x7c
[42993.140954] [<ffffffff>] 0xffffffff
[42993.140958]
[42993.140959] -> #1 (&mm->mmap_sem){----}:
[42993.140964] [<c0162845>] __lock_acquire+0x9a6/0xb19
[42993.140968] [<c0162a13>] lock_acquire+0x5b/0x81
[42993.140972] [<c019f0f7>] might_fault+0x65/0x85
[42993.140975] [<c02dafc1>] copy_to_user+0x31/0x100
[42993.140980] [<c01c1248>] filldir64+0x9c/0xd2
[42993.140984] [<c01f6e78>] sysfs_readdir+0x11c/0x150
[42993.140988] [<c01c1469>] vfs_readdir+0x6d/0x99
[42993.140992] [<c01c14fd>] sys_getdents64+0x68/0xa5
[42993.140996] [<c0117ff6>] syscall_call+0x7/0xb
[42993.141000] [<ffffffff>] 0xffffffff
[42993.141009]
[42993.141009] -> #0 (sysfs_mutex){--..}:
[42993.141014] [<c016271a>] __lock_acquire+0x87b/0xb19
[42993.141018] [<c0162a13>] lock_acquire+0x5b/0x81
[42993.141022] [<c04c402a>] __mutex_lock_common+0xdc/0x33a
[42993.141026] [<c04c432f>] mutex_lock_nested+0x33/0x3b
[42993.141030] [<c01f7098>] sysfs_addrm_start+0x28/0x9a
[42993.141034] [<c01f7522>] sysfs_remove_dir+0x77/0xab
[42993.141039] [<c02d5a0b>] kobject_del+0xf/0x2c
[42993.141042] [<c02d5b5c>] kobject_release+0x134/0x1cb
[42993.141046] [<c02d68c0>] kref_put+0x3c/0x4a
[42993.141050] [<c02d59a4>] kobject_put+0x37/0x3c
[42993.141054] [<c021cc89>] ext4_put_super+0xab/0x215
[42993.141058] [<c01b7f55>] generic_shutdown_super+0x62/0xe3
[42993.141062] [<c01b7ff8>] kill_block_super+0x22/0x36
[42993.141066] [<c01b80c3>] deactivate_super+0x5c/0x6f
[42993.141070] [<c01c8b98>] mntput_no_expire+0xd4/0x106
[42993.141074] [<c01c909c>] sys_umount+0x2a1/0x2c6
[42993.141078] [<c01c90d3>] sys_oldumount+0x12/0x14
[42993.141082] [<c0117ff6>] syscall_call+0x7/0xb
[42993.141086] [<ffffffff>] 0xffffffff
[42993.141092]
[42993.141093] other info that might help us debug this:
[42993.141094]
[42993.141097] 2 locks held by umount/30941:
[42993.141099] #0: (&type->s_umount_key#30){----}, at: [<c01b80be>] deactivate_super+0x57/0x6f
[42993.141107] #1: (&type->s_lock_key#6){--..}, at: [<c01b7493>] lock_super+0x26/0x28
[42993.141115]

So if I'm reading this right, the problem is happening because
someplace in the kernel, fs/sysfs code is grabbing mmap_sem while it
is holding sysfs_mutex, right? But I can't see where that might be
happening...

- Ted
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