Re: [PATCH v2 23/54] fs: use refcount_inc_not_zero in igrab

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Aug 28, 2025 at 10:08:06PM +0000, Eric Biggers wrote:
> On Tue, Aug 26, 2025 at 11:39:23AM -0400, Josef Bacik wrote:
> > +static inline struct inode *inode_tryget(struct inode *inode)
> > +{
> > +	/*
> > +	 * We are using inode_tryget() because we're interested in getting a
> > +	 * live reference to the inode, which is ->i_count. Normally we would
> > +	 * grab i_obj_count first, as it is the higher priority reference.
> > +	 * However we're only interested in making sure we have a live inode,
> > +	 * and we know that if we get a reference for i_count then we can safely
> > +	 * acquire i_obj_count because we always drop i_obj_count after dropping
> > +	 * an i_count reference.
> > +	 *
> > +	 * This is meant to be used either in a place where we have an existing
> > +	 * i_obj_count reference on the inode, or under rcu_read_lock() so we
> > +	 * know we're safe in accessing this inode still.
> > +	 */
> > +	VFS_WARN_ON_ONCE(!iobj_count_read(inode) && !rcu_read_lock_held());
> > +
> > +	if (refcount_inc_not_zero(&inode->i_count)) {
> > +		iobj_get(inode);
> > +		return inode;
> > +	}
> > +
> > +	/*
> > +	 * If we failed to increment the reference count, then the
> > +	 * inode is being freed or has been freed.  We return NULL
> > +	 * in this case.
> > +	 */
> > +	return NULL;
> 
> Is there a reason to take one i_obj_count reference per i_count
> reference, instead of a single i_obj_count reference associated with
> i_count being nonzero?  With a single reference owned by i_count != 0,
> it wouldn't be necessary to touch i_obj_count when i_count is changed,
> except when i_count reaches zero.  That would be more efficient.
> 
> BTW, fscrypt_master_key::mk_active_refs and
> fscrypt_master_key::mk_struct_refs use that solution.  For
> mk_active_refs != 0, one reference in mk_struct_refs is held.
> 

That certainly could be done as well, hell I do that pattern for the writeback
lists and such. I'll discuss with Christian and see what he thinks. Thanks,

Josef