Re: 3.1.0-rc3 -- INFO: possible circular locking dependency detected

[Dave Chinner]

On Tue, Aug 23, 2011 at 07:59:20AM -0400, Josh Boyer wrote:
> On Tue, Aug 23, 2011 at 7:49 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> >> >  Possible unsafe locking scenario:
> >> >
> >> >       CPU0                    CPU1
> >> >       ----                    ----
> >> >  lock(&mm->mmap_sem);
> >> >                               lock(&sb->s_type->i_mutex_key);
> >> >                               lock(&mm->mmap_sem);
> >> >  lock(&sb->s_type->i_mutex_key);
> >> >
> >> >  *** DEADLOCK ***
> >>
> >> This one was reported yesterday: https://lkml.org/lkml/2011/8/21/163
> >> and we're hoping Ted (or someone else from the ext4 camp) can comment
> >> on why ext4_evict_inode is holding i_mutex.
> >
> > Actually, the problem has nothing to do with ext4. the problem is
> > that remove_vma() is holding the mmap_sem while calling fput(). The
> > correct locking order is i_mutex->mmap_sem, as documented in
> > mm/filemap.c:
> >
> >  *  ->i_mutex                   (generic_file_buffered_write)
> >  *    ->mmap_sem                (fault_in_pages_readable->do_page_fault)
> >
> >
> > The way remove_vma() calls fput() also triggers lockdep reports in
> > XFS and it will do so with any filesystem that takes an inode
> > specific lock in it's evict() processing. IOWs, remove_vma() needs
> > fixing, not ext4....
> 
> Er... ok.  So the remove_vma code hasn't changed since 2008.  We're
> only seeing this issue now because the debugging code has improved,
> or?

The problem has been there since at least 2008.  Here's an early
XFS report from 2.6.24:

http://oss.sgi.com/archives/xfs/2008-02/msg00931.html

Here's an XFS report
to match the ext4 one in this thread from 2009:

http://oss.sgi.com/archives/xfs/2009-03/msg00149.html

You won't find reports much older than this - it only started to be
reported when lockdep support in XFS matured and it started to be
widely used....

> At any rate, the proposed solution is to make remove_vma drop mmap_sem
> before calling fput, or make it not call fput, or?

Ask the VM folk - this is the only response I can remember from them
is this:

http://oss.sgi.com/archives/xfs/2009-03/msg00224.html

Maybe now that ext4 is hitting the problem something will be done
about it...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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