Re: [PATCH 3/5] cifsd: add file operations

[Al Viro]

On Mon, Mar 22, 2021 at 02:13:42PM +0900, Namjae Jeon wrote:

> +static struct ksmbd_file *__ksmbd_lookup_fd(struct ksmbd_file_table *ft,
> +					    unsigned int id)
> +{
> +	bool unclaimed = true;
> +	struct ksmbd_file *fp;
> +
> +	read_lock(&ft->lock);
> +	fp = idr_find(ft->idr, id);
> +	if (fp)
> +		fp = ksmbd_fp_get(fp);
> +
> +	if (fp && fp->f_ci) {
> +		read_lock(&fp->f_ci->m_lock);
> +		unclaimed = list_empty(&fp->node);
> +		read_unlock(&fp->f_ci->m_lock);
> +	}
> +	read_unlock(&ft->lock);
> +
> +	if (fp && unclaimed) {
> +		atomic_dec(&fp->refcount);
> +		return NULL;
> +	}

Can that atomic_dec() end up dropping the last remaining reference?
If not, what's to prevent that?