VFS scalability + cgroups == panic

[Eric Paris]

[   46.337486] systemd[1]: readahead-collect.service: main process exited, code=exited, status=1
[   46.339544] BUG: unable to handle kernel NULL pointer dereference at           (null)
[   46.340232] IP: [<          (null)>]           (null)
[   46.340232] PGD 3a885067 PUD 3b2ad067 PMD 0
[   46.340232] Oops: 0010 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[   46.340232] last sysfs file: /sys/module/ipv6/parameters/disable
[   46.340232] CPU 1
[   46.340232] Modules linked in: ipv6 autofs4 ext4 jbd2 crc16 usbhid
[   46.340232]
[   46.340232] Pid: 1, comm: systemd Tainted: G        W   2.6.37-rc6-kernel1-next-20101220+ #149 /KVM
[   46.340232] RIP: 0010:[<0000000000000000>]  [<          (null)>]           (null)
[   46.340232] RSP: 0018:ffff88003de09d40  EFLAGS: 00010206
[   46.340232] RAX: ffffffff81612080 RBX: ffff88003d8fdd20 RCX: 0000000000000000
[   46.340232] RDX: ffffffff8154d737 RSI: ffff88003d8fdd98 RDI: ffff88003d8fdd20
[   46.340232] RBP: ffff88003de09d78 R08: 0000000000000000 R09: 0000000000000000
[   46.340232] R10: 0000000000000001 R11: 0000000000000001 R12: ffff88003d8fdd80
[   46.340232] R13: ffff88003d8fd300 R14: ffff88003d8fd388 R15: ffff88003d8fddf8
[   46.340232] FS:  00007f53e84b87c0(0000) GS:ffff88003e200000(0000) knlGS:0000000000000000
[   46.340232] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   46.340232] CR2: 0000000000000000 CR3: 000000003b21f000 CR4: 00000000000006e0
[   46.340232] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   46.340232] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[   46.340232] Process systemd (pid: 1, threadinfo ffff88003de08000, task ffff88003de00000)
[   46.340232] Stack:
[   46.340232]  ffffffff81146570 ffff88003de09d78 ffff88003d8fdd20 ffff88003d8fd2a0
[   46.340232]  ffff88003d8fd300 ffff88003d8fd388 ffff88003d8fddf8 ffff88003de09dc8
[   46.340232]  ffffffff810ad273 ffff88003de09da8 ffff88003d8fdd80 ffff88003ac0b7c8
[   46.340232] Call Trace:
[   46.340232]  [<ffffffff81146570>] ? dput+0xd0/0x220
[   46.340232]  [<ffffffff810ad273>] cgroup_clear_directory+0x93/0x130
[   46.340232]  [<ffffffff810af7f6>] cgroup_rmdir+0x3b6/0x550
[   46.340232]  [<ffffffff8107d720>] ? autoremove_wake_function+0x0/0x40
[   46.340232]  [<ffffffff8113ac14>] vfs_rmdir+0xa4/0xe0
[   46.340232]  [<ffffffff8114cd0d>] ? mnt_want_write+0x4d/0x90
[   46.340232]  [<ffffffff8113d18b>] do_rmdir+0x10b/0x120
[   46.340232]  [<ffffffff81093188>] ? lockdep_sys_exit+0x28/0x80
[   46.340232]  [<ffffffff8154d479>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[   46.340232]  [<ffffffff8113d1f1>] sys_rmdir+0x11/0x20
[   46.340232]  [<ffffffff8100bf12>] system_call_fastpath+0x16/0x1b

This is a linux-next kernel.  I'm running systemd which actually makes
use of cgroups and the system panics pretty early in the boot process.
I decided to bisect this down and found that BAD=d99d659c52cbca98 and
GOOD=741fbe01b622eda8c75044 which says it is the VFS scalability patches
causing the problems.  I would try to bisect even further but the patch
set is not bisect safe.  I find that at least commits 065d2abddcc69a225
and 11beda425c8f2c529d02 won't build due to the use of
dget_locked_dlock() being undefined (both OCFS2 and CONFIGFS)

I'm still trying to bisect around the brokenness, but hopefully you
already know or can point out the problem?

-Eric

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