syscall rmdir hangs with autofs

[Sebastian Hetze <s.hetze@linux-ag.com>]

Hi *,

we are encountering occasional problems with autofs running inside
an KVM guest.

[1387441.969106] INFO: task automount:26560 blocked for more than 120 seconds.
[1387441.969110] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[1387441.969112] automount     D e8510198     0 26560   2702 0x00000000
[1387441.969117]  db0a1ef4 00000082 80000000 e8510198 0004ed69 c8266000 f6e85a40 00000000
[1387441.969123]  c08455e0 c08455e0 f41157f0 f4115a88 c55315e0 00000000 c0207c0a db0a1ef0
[1387441.969128]  f4115a88 f7222bbc f7222bb8 ffffffff db0a1f20 c05976ae db0a1f14 f41157f0
[1387441.969133] Call Trace:
[1387441.969140]  [<c0207c0a>] ? mntput_no_expire+0x1a/0xd0
[1387441.969146]  [<c05976ae>] __mutex_lock_slowpath+0xbe/0x120
[1387441.969149]  [<c05975d0>] mutex_lock+0x20/0x40
[1387441.969152]  [<c01fbc82>] do_rmdir+0x52/0xe0
[1387441.969155]  [<c059ae47>] ? do_page_fault+0x1d7/0x3a0
[1387441.969158]  [<c01fbd70>] sys_rmdir+0x10/0x20
[1387441.969161]  [<c01033cc>] syscall_call+0x7/0xb

The block always occurs in sys_rmdir when automount tries to remove the
mountpoint right after umounting the filesystem. There is an successful lstat()
on the mountpoint directly precceeding the rmdir call. 

It looks like we are triggering some sort of race condition here.

We are currently using 2.6.31-20-generic-pae ubuntu kernel in the 6 CPU guest,
2.6.34 vanilla and qemu-kvm-0.12.4 in the host. But the problem existed
long before with all different combinations of guest/host/qemu versions.
The virtual HD is if=ide,format=host_device,cache=none on an DRBD container
on top of an LVM device. FS is ext3.

Unfortunately, the problem is not easy reproduceable. It occurs every one
or two weeks. But since the hanging system call blocks the whole filesystem
we have to reboot the guest to get it into an useable state again.

Any ideas what's going wrong here?

Best regards,

  Sebastian