Re: [PATCH 13/18] fs: split locking of inode writeback and LRU lists

Nick Piggin
2010-10-16T07:57:16Z

On Fri, Oct 08, 2010 at 04:18:16AM -0400, Christoph Hellwig wrote:
> On Fri, Oct 08, 2010 at 07:00:18PM +1100, Dave Chinner wrote:
> > > >
> > > > - if (inode->i_state & (I_NEW | I_WILL_FREE)) {
> > > > + if (inode->i_state & (I_NEW | I_WILL_FREE | I_FREEING)) {
> > > > requeue_io(inode);
> > > > continue;
> > > > }
> > >
> > > What does this have to do with the rest of the patch?
> >
> > That's because there's now a window between setting I_FREEING and taking
> > the inode off the writeback list which means that we can see inodes
> > in that state here. Generally it means that the code setting
> > I_FREEING is spinning waiting for the wb->b_lock that this thread
> > currently holds so it can be removed from the list.. Hence the requeue
> > to move the inode out of the way and keep processing inodes for
> > writeback.
>
> That needs some documentation both in the changelog and in the code
> I think.

This is another instance where the irregular i_lock locking is making
these little subtleties to the locking. I think that is actually much
worse for maintainence/complexity than a few trylocks which can be
mostly removed with rcu anyway (which are obvious because of the well
documented lock order).