Re: [PATCH 12/14] nfsd4: remove use of mutex for file_hashtable

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Mar 18, 2009 at 10:40:41PM +0200, Benny Halevy wrote:
> On Mar. 18, 2009, 22:28 +0200, "J. Bruce Fields" <bfields@fieldses.org> wrote:
> >  static inline void
> >  put_nfs4_file(struct nfs4_file *fi)
> >  {
> > -	kref_put(&fi->fi_ref, free_nfs4_file);
> 
> missing spin_lock(&recall_lock);?
> 
> 
> > +	if (atomic_dec_and_lock(&fi->fi_ref, &recall_lock)) {
> > +		list_del(&fi->fi_hash);
> > +		spin_unlock(&recall_lock);
> > +		iput(fi->fi_inode);
> > +		kmem_cache_free(file_slab, fi);
> > +	}
> 
> 	} else {
> 		spin_unlock(&recall_lock);
> 	}
> 
> or am I missing something?

>From include/linux/spinlock.h:

/**
 * atomic_dec_and_lock - lock on reaching reference count zero
 * @atomic: the atomic counter
 * @lock: the spinlock in question
 *
 * Decrements @atomic by 1.  If the result is 0, returns true and locks
 * @lock.  Returns false for all other cases.
 */

So it's useful for cases such as this, when a reference-counted object
is reachable from some data structure visible to others, and you want to
remove it from that structure on dropping the last reference (as opposed
to keeping one reference for the structure itself, and requiring the
object to be explicitly removed).

(Unless I'm missing something else.)

--b.