Re: [PATCH 09/18] fs: rework icount to be a locked variable

Dave Chinner
2010-10-08T07:50:11Z

On Fri, Oct 08, 2010 at 03:27:49AM -0400, Christoph Hellwig wrote:
> > index 2953e9f..9f04478 100644
> > --- a/fs/btrfs/inode.c
> > +++ b/fs/btrfs/inode.c
> > @@ -1964,8 +1964,14 @@ void btrfs_add_delayed_iput(struct inode *inode)
> > struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
> > struct delayed_iput *delayed;
> >
> > - if (atomic_add_unless(&inode->i_count, -1, 1))
> > + /* XXX: filesystems should not play refcount games like this */
> > + spin_lock(&inode->i_lock);
> > + if (inode->i_ref > 1) {
> > + inode->i_ref--;
> > + spin_unlock(&inode->i_lock);
> > return;
> > + }
> > + spin_unlock(&inode->i_lock);
>
> Yeah, all that i_count/i_ref mess in btrfs needs some serious work.
> Chris?
>
> > +
> > +/*
> > + * inode_lock must be held
> > + */
> > +void iref_locked(struct inode *inode)
> > +{
> > + inode->i_ref++;
> > +}
> > EXPORT_SYMBOL_GPL(iref_locked);
>
> I'm a big fan of _GPL exports, but adding this for a trivial counter
> increment seems a bit weird.

OK, will drop the _GPL.
>
> > int iref_read(struct inode *inode)
> > {
> > - return atomic_read(&inode->i_count);
> > + int ref;
> > +
> > + spin_lock(&inode->i_lock);
> > + ref = inode->i_ref;
> > + spin_unlock(&inode->i_lock);
> > + return ref;
> > }
>
> There's no need to lock a normal 32-bit variable for readers.

Ok, but will need a memory barrier instead?

>
> > + inode->i_ref--;
> > + if (inode->i_ref == 0) {
>
> if (--inode->i_ref == 0) {
>
> might be a bit more idiomatic.

OK.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com