Btrfs devel - Oct 2010 - Re: [PATCH 09/18] fs: rework icount to be a locked variable

> 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. 
>  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.
> +		inode->i_ref--;
> +		if (inode->i_ref == 0) {
		if (--inode->i_ref == 0) {

might be a bit more idiomatic.

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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

On Fri, Oct 08, 2010 at 06:50:01PM +1100, Dave Chinner
wrote:
> > There''s no need to lock a normal 32-bit variable for readers.
> 
> Ok, but will need a memory barrier instead?
Isn''t spin_unlock supposed to be one?  I''ll need some of the
locking
experts to shime in.

On Fri, Oct 08, 2010 at 10:17:14AM +0200, Christoph Hellwig
wrote:
> On Fri, Oct 08, 2010 at 06:50:01PM +1100, Dave Chinner wrote:
> > > There''s no need to lock a normal 32-bit variable for
readers.
> > 
> > Ok, but will need a memory barrier instead?
> 
> Isn''t spin_unlock supposed to be one?  I''ll need some of
the locking
> experts to shime in.
Not really a locking expert, but the locking operations are supposed to
have an implicit barrier.

-chris

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