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Date: Mon, 05 Jan 2009 11:23:33 +0800
From: Li Zefan <lizf@cn.fujitsu.com>
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CC: Andrew Morton <akpm@linux-foundation.org>, Paul Menage <menage@google.com>,
       Al Viro <viro@ZenIV.linux.org.uk>
Subject: [cgroup or VFS ?]  WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()
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Thread 1:
  for ((; ;))
  {
      mount -t cgroup -o cpuset xxx /mnt > /dev/null 2>&1
      mkdir /mnt/0 > /dev/null 2>&1
      rmdir /mnt/0 > /dev/null 2>&1
      umount /mnt > /dev/null 2>&1
  }

Thread 2:
  for ((; ;))
  {
      mount -t cpuset xxx /mnt > /dev/null 2>&1
      umount /mnt > /dev/null 2>&1
  }

(Note: Again it is irrelevant which cgroup subsys is used.)

After a while this showed up:

------------[ cut here ]------------
WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()
Hardware name: Aspire SA85
Modules linked in: bridge stp llc autofs4 dm_mirror dm_region_hash dm_log dm_mod r8169 parport_pc mii parport sg button sata_sis pata_sis ata_generic libata sd_mod scsi_mod ext3 jbd mbcache uhci_hcd ohci_hcd ehci_hcd [last unloaded: scsi_wait_scan]
Pid: 4745, comm: umount Not tainted 2.6.28 #479
Call Trace:
 [<c042bbe3>] warn_slowpath+0x79/0x8f
 [<c044babf>] ? __lock_acquire+0x69a/0x700
 [<c04ae44e>] ? mntput_no_expire+0x79/0xf2
 [<c04ae481>] mntput_no_expire+0xac/0xf2
 [<c04ae968>] sys_umount+0x26a/0x2b1
 [<c04ae9c1>] sys_oldumount+0x12/0x14
 [<c0403251>] sysenter_do_call+0x12/0x31
---[ end trace 79d0ab4bef01333f ]---

The WARNING is: WARN_ON(atomic_read(&mnt->__mnt_writers));