linux-kernel - Re: [PATCH] vfs: Fix lock inversion in drop_pagecache

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Date:	Wed, 26 Mar 2008 10:31:38 +0100
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	dgc@....com, wfg@...l.ustc.edu.cn, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfs: Fix lock inversion in drop_pagecache_sb()

On Tue 25-03-08 12:53:54, Andrew Morton wrote:
> On Tue, 25 Mar 2008 19:12:27 +0100
> Jan Kara <jack@...e.cz> wrote:
> 
> > Fix longstanding lock inversion in drop_pagecache_sb by dropping inode_lock
> > before calling __invalidate_mapping_pages(). We just have to make sure
> > inode won't go away from under us by keeping reference to it and putting
> > the reference only after we have safely resumed the scan of the inode
> > list. A bit tricky but not too bad...
> > 
> > Signed-off-by: Jan Kara <jack@...e.cz>
> > CC: Fengguang Wu <wfg@...l.ustc.edu.cn>
> > CC: David Chinner <dgc@....com>
> > 
> > ---
> >  fs/drop_caches.c |    8 +++++++-
> >  1 files changed, 7 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/drop_caches.c b/fs/drop_caches.c
> > index 59375ef..f5aae26 100644
> > --- a/fs/drop_caches.c
> > +++ b/fs/drop_caches.c
> > @@ -14,15 +14,21 @@ int sysctl_drop_caches;
> >  
> >  static void drop_pagecache_sb(struct super_block *sb)
> >  {
> > -	struct inode *inode;
> > +	struct inode *inode, *toput_inode = NULL;
> >  
> >  	spin_lock(&inode_lock);
> >  	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
> >  		if (inode->i_state & (I_FREEING|I_WILL_FREE))
> >  			continue;
> 
> OT: it might be worth having an `if (mapping->nrpages==0) continue' here.
  Good idea. I'll send a patch in a minute.

> > +		__iget(inode);
> > +		spin_unlock(&inode_lock);
> >  		__invalidate_mapping_pages(inode->i_mapping, 0, -1, true);
> > +		iput(toput_inode);
> > +		toput_inode = inode;
> > +		spin_lock(&inode_lock);
> >  	}
> >  	spin_unlock(&inode_lock);
> > +	iput(toput_inode);
> >  }
> >  
> >  void drop_pagecache(void)
> 
> hrm.  So we have a random ref on an inode without holding inode_lock.  If
> we race with invalidate_list() we end up with an inode stuck on s_inodes
> and "Self-destruct in 5 seconds.  Have a nice day...", don't we?
  We hold s_umount for reading so we should be safe against someone trying
to do umount. We could possibly race with invalidate_list() called from
check_disk_change() but removing media without unmounting is a bad behavior
anyway. So I think we are fine.

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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