hugetlb locking bug.

[Dave Jones]

Just hit this lockdep report..

	Dave

 =======================================================
 [ INFO: possible circular locking dependency detected ]
 2.6.39-rc3+ #3
 -------------------------------------------------------
 trinity/11299 is trying to acquire lock:
  (&sb->s_type->i_mutex_key#18){+.+.+.}, at: [<ffffffff811ebfec>] hugetlbfs_file_mmap+0x7f/0x10d
 
 but task is already holding lock:
  (&mm->mmap_sem){++++++}, at: [<ffffffff81109097>] sys_mmap_pgoff+0xf8/0x16a
 
 which lock already depends on the new lock.
 
 
 the existing dependency chain (in reverse order) is:
 
 -> #1 (&mm->mmap_sem){++++++}:
        [<ffffffff81086c59>] lock_acquire+0xd0/0xfb
        [<ffffffff81100cd0>] might_fault+0x89/0xac
        [<ffffffff8114159b>] filldir+0x6f/0xc7
        [<ffffffff81150075>] dcache_readdir+0x67/0x204
        [<ffffffff811417ed>] vfs_readdir+0x78/0xb1
        [<ffffffff8114190c>] sys_getdents+0x7e/0xd1
        [<ffffffff814c22c2>] system_call_fastpath+0x16/0x1b
 
 -> #0 (&sb->s_type->i_mutex_key#18){+.+.+.}:
        [<ffffffff810864e6>] __lock_acquire+0x99e/0xc81
        [<ffffffff81086c59>] lock_acquire+0xd0/0xfb
        [<ffffffff814b9d00>] __mutex_lock_common+0x4c/0x35b
        [<ffffffff814ba0d3>] mutex_lock_nested+0x3e/0x43
        [<ffffffff811ebfec>] hugetlbfs_file_mmap+0x7f/0x10d
        [<ffffffff81108ac8>] mmap_region+0x26d/0x446
        [<ffffffff81108f45>] do_mmap_pgoff+0x2a4/0x2fe
        [<ffffffff811090b7>] sys_mmap_pgoff+0x118/0x16a
        [<ffffffff8100c56c>] sys_mmap+0x22/0x24
        [<ffffffff814c22c2>] system_call_fastpath+0x16/0x1b
 
 other info that might help us debug this:
 
 1 lock held by trinity/11299:
  #0:  (&mm->mmap_sem){++++++}, at: [<ffffffff81109097>] sys_mmap_pgoff+0xf8/0x16a
 
 stack backtrace:
 Pid: 11299, comm: trinity Not tainted 2.6.39-rc3+ #3
 Call Trace:
  [<ffffffff814b1126>] print_circular_bug+0xa6/0xb5
  [<ffffffff810864e6>] __lock_acquire+0x99e/0xc81
  [<ffffffff811ebfec>] ? hugetlbfs_file_mmap+0x7f/0x10d
  [<ffffffff81086c59>] lock_acquire+0xd0/0xfb
  [<ffffffff811ebfec>] ? hugetlbfs_file_mmap+0x7f/0x10d
  [<ffffffff811ebfec>] ? hugetlbfs_file_mmap+0x7f/0x10d
  [<ffffffff814b9d00>] __mutex_lock_common+0x4c/0x35b
  [<ffffffff811ebfec>] ? hugetlbfs_file_mmap+0x7f/0x10d
  [<ffffffff814b3ee2>] ? __slab_alloc+0xdb/0x3b7
  [<ffffffff81087043>] ? trace_hardirqs_on_caller+0x10b/0x12f
  [<ffffffff814b3eeb>] ? __slab_alloc+0xe4/0x3b7
  [<ffffffff81108a12>] ? mmap_region+0x1b7/0x446
  [<ffffffff814ba0d3>] mutex_lock_nested+0x3e/0x43
  [<ffffffff811ebfec>] hugetlbfs_file_mmap+0x7f/0x10d
  [<ffffffff81108ac8>] mmap_region+0x26d/0x446
  [<ffffffff81108f45>] do_mmap_pgoff+0x2a4/0x2fe
  [<ffffffff811090b7>] sys_mmap_pgoff+0x118/0x16a
  [<ffffffff81087043>] ? trace_hardirqs_on_caller+0x10b/0x12f
  [<ffffffff8100c56c>] sys_mmap+0x22/0x24
  [<ffffffff814c22c2>] system_call_fastpath+0x16/0x1b

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