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.
- Eric
