Re: [patch 29/52] fs: icache lock i_count

Dave Chinner
2010-07-01T02:36:35Z

On Wed, Jun 30, 2010 at 10:05:02PM +1000, Nick Piggin wrote:
> On Wed, Jun 30, 2010 at 05:27:02PM +1000, Dave Chinner wrote:
> > On Thu, Jun 24, 2010 at 01:02:41PM +1000, npiggin@suse.de wrote:
> > > Protect inode->i_count with i_lock, rather than having it atomic.
> > > Next step should also be to move things together (eg. the refcount increment
> > > into d_instantiate, which will remove a lock/unlock cycle on i_lock).
> > .....
> > > Index: linux-2.6/fs/inode.c
> > > ===================================================================
> > > --- linux-2.6.orig/fs/inode.c
> > > +++ linux-2.6/fs/inode.c
> > > @@ -33,14 +33,13 @@
> > > * inode_hash_lock protects:
> > > * inode hash table, i_hash
> > > * inode->i_lock protects:
> > > - * i_state
> > > + * i_state, i_count
> > > *
> > > * Ordering:
> > > * inode_lock
> > > * sb_inode_list_lock
> > > * inode->i_lock
> > > - * inode_lock
> > > - * inode_hash_lock
> > > + * inode_hash_lock
> > > */
> >
> > I thought that the rule governing the use of inode->i_lock was that
> > it can be used anywhere as long as it is the innermost lock.
> >
> > Hmmm, no references in the code or documentation. Google gives a
> > pretty good reference:
> >
> > http://www.mail-archive.com/linux-ext4@vger.kernel.org/msg02584.html
> >
> > Perhaps a different/new lock needs to be used here?
>
> Well I just changed the order (and documented it to boot :)). It's
> pretty easy to verify that LOR is no problem. inode hash is only
> taken in a very few places so other code outside inode.c is fine to
> use i_lock as an innermost lock.

It's not just the inode_hash_lock - you move four or five other
locks under inode->i_lock as the series progresses. IOWs, there's
now many paths and locking orders where the i_lock is not innermost.
If we go forward with this, it's only going to get more complex and
eventually somewhere we'll need a new lock for an innermost
operation because inode->i_lock is no longer safe to use....

Seriously: use a new lock for high level inode operations you are
optimising - don't repurpose an existing lock with different usage
rules just because it's convenient.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com