Re: [PATCH] Btrfs: put delayed iput tracking list inside in-memory inode

[Liu Bo <bo.li.liu@oracle.com>]

On Wed, Dec 12, 2012 at 09:47:11AM -0500, Josef Bacik wrote:
> On Tue, Dec 11, 2012 at 06:50:40PM -0700, Liu Bo wrote:
> > On Tue, Dec 11, 2012 at 09:57:50AM -0500, Josef Bacik wrote:
> > > On Fri, Nov 30, 2012 at 04:24:43AM -0700, Liu Bo wrote:
> > > > This can save us a dynamic memory allocation/free.
> > > > 
> > > 
> > > You can have multiple outstanding delayed iputs per inode, so this will result
> > > in inodes still being in use on unmount, so this isn't going to work.
> > 
> > Yeah, you're right, but what if we add a check
> > 
> > if (list_empty(&bi->iput_list))
> > 	list_add(&bi->iput_list, &fs_info->iput_list));
> > 
> > Am I missing something?
> > 
> 
> Yes, we have the delayed iput stuff to keep us from running the actual iput
> stuff where we are for locking reasons.  If we do the if (list_empty) check then
> we have to do something for the case wehre the list isn't empty so we don't leak
> references to the inode, which would basically mean doing an iput which is what
> we don't want to do.  We could do a hybrid approach where we do your list and
> then allocate if we're already on the list, or add a counter so we know how many
> references to drop.  But this as it is won't work.  (Keep in mind I've been
> buried in tree logging for 3 months so if I'm wrong bear with me).  Thanks,

I got your point, I'll add a counter here to tracking the dropped reference per
inode.

Mainly I was thinking memory allocation/free is just way too expensive. 

Thanks for reviewing it.

thanks,
liubo