Re: lockdep wierdness...

[Peter Zijlstra]

On Mon, 2007-09-24 at 22:13 -0400, Steven Rostedt wrote:
> On Mon, Sep 24, 2007 at 06:07:38PM -0400, Trond Myklebust wrote:
> > I'm seeing lockdep warning about a potential lock inversion between
> > &mm->mmap_sem and &inode->i_mutex in NFS (see attachment).
> > 
> > Unfortunately the basis for the warning appears to be the behaviour in
> > ext3(???). AFAICS there is no way for NFS to share an inode->i_mutex
> > with ext3. What to do?
> 
> Actually this can probably happen just on NFS alone.
> 
> > 
> > Trond
> 
> > =======================================================
> > [ INFO: possible circular locking dependency detected ]
> > 2.6.23-rc7-g8809e921 #1
> > -------------------------------------------------------
> > beagle-build-in/24375 is trying to acquire lock:
> >  (&mm->mmap_sem){----}, at: [<c05a2887>] do_page_fault+0x17d/0x591
> > 
> > but task is already holding lock:
> >  (&inode->i_mutex){--..}, at: [<c059f9e3>] mutex_lock+0x1c/0x1f
> > 
> > which lock already depends on the new lock.
> > 
> > 
> > the existing dependency chain (in reverse order) is:
> > 
> > -> #1 (&inode->i_mutex){--..}:
> >        [<c043d4da>] __lock_acquire+0x9f3/0xba6
> >        [<c043da62>] lock_acquire+0x5f/0x78
> >        [<c059f832>] __mutex_lock_slowpath+0xe5/0x27a
> >        [<c059f9e3>] mutex_lock+0x1c/0x1f
> >        [<f8c92495>] nfs_revalidate_mapping+0x64/0x9c [nfs]
> >        [<f8c8ff2a>] nfs_file_mmap+0x46/0x75 [nfs]
> >        [<c046097c>] mmap_region+0x1ea/0x3b8
> >        [<c0460e9b>] do_mmap_pgoff+0x27b/0x2da
> >        [<c0407d77>] sys_mmap2+0x9b/0xb5
> >        [<c040405e>] sysenter_past_esp+0x5f/0x99
> >        [<ffffffff>] 0xffffffff
> > 
> > -> #0 (&mm->mmap_sem){----}:
> >        [<c043d3c6>] __lock_acquire+0x8df/0xba6
> >        [<c043da62>] lock_acquire+0x5f/0x78
> >        [<c04360db>] down_read+0x3a/0x4c
> >        [<c05a2887>] do_page_fault+0x17d/0x591
> >        [<c05a1382>] error_code+0x72/0x78
> >        [<f88acaac>] call_filldir+0xac/0xc3 [ext3]
> >        [<f88acdb2>] ext3_readdir+0x217/0x5e5 [ext3]
> >        [<c04798a1>] vfs_readdir+0x67/0x93
> >        [<c0479af6>] sys_getdents+0x5f/0x9d
> >        [<c040405e>] sysenter_past_esp+0x5f/0x99
> >        [<ffffffff>] 0xffffffff
> 
> The circular lock seems to be this:
> 
> #1:
> 
>   sys_mmap2:              down_write(&mm->mmap_sem);
>   nfs_revalidate_mapping: mutex_lock(&inode->i_mutex);
> 
> 
> #0:
> 
>   vfs_readdir:     mutex_lock(&inode->i_mutex);
>    - during the readdir (filldir64), we take a user fault (missing page?)
>     and call do_page_fault -
>   do_page_fault:   down_read(&mm->mmap_sem);
> 
> 
> So it does indeed look like a circular locking. Now the question is, "is
> this a bug?".  Looking like the inode of #1 must be a file or something
> else that you can mmap and the inode of #0 seems it must be a directory.
> I would say "no".
> 
> Now if you can readdir on a file or mmap a directory, then this could be
> an issue.
> 
> Otherwise, I'd love to see someone teach lockdep about this issue! ;-)

Christoph,

does Steve's story make sense? 

If so, do we know at alloc_inode() time what type of inode we're
requesting; file or dir. Again, if so, the lockdep annotation should be
trivial in the light of the recently merged patch:

 + lockdep-give-each-filesystem-its-own-inode-lock-class.patch

All that would need to be done is add an extra lock_class_key to
file_system_type for i_mutex_dir_key, and extend alloc_inode to say
something like:

  if (dir)
    lockdep_set_class(&inode->i_mutex, &sb->s_type->i_mutex_dir_key);
  else
    lockdep_set_class(&inode->i_mutex, &sb->s_type->i_mutex_key);


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