linux-kernel - Re: [patch 29/52] fs: icache lock i

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Date:	Wed, 30 Jun 2010 22:05:02 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>
Subject: Re: [patch 29/52] fs: icache lock i_count

On Wed, Jun 30, 2010 at 05:27:02PM +1000, Dave Chinner wrote:
> On Thu, Jun 24, 2010 at 01:02:41PM +1000, npiggin@...e.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.

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