Re: [PATCH v11 1/5] fs: add iput_if_not_last() helper

[Mateusz Guzik <mjguzik@gmail.com>]

On Mon, Jun 29, 2026 at 07:08:44PM +0800, Yun Zhou wrote:
> Add a helper that drops an inode reference only if the caller does not
> hold the last one.  Returns true if the reference was dropped, false
> otherwise.
> 
> This is useful for filesystems that need to release inode references
> in contexts where triggering final iput (and thus eviction) would be
> unsafe due to lock ordering constraints.  The caller can check the
> return value and defer the final iput to a safe context.
> 
> Unlike iput_not_last() which BUG_ON's if called with the last ref,
> this variant is designed to be called speculatively.
> 
> Signed-off-by: Yun Zhou <yun.zhou@windriver.com>
> Suggested-by: Jan Kara <jack@suse.cz>
> Reviewed-by: Jan Kara <jack@suse.cz>
> ---
>  include/linux/fs.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index d10897b3a1e3..04f0de78fa7a 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2413,6 +2413,19 @@ static inline void super_set_sysfs_name_generic(struct super_block *sb, const ch
>  extern void ihold(struct inode * inode);
>  extern void iput(struct inode *);
>  void iput_not_last(struct inode *);
> +
> +/**
> + * iput_if_not_last - drop an inode reference only if it is not the last one
> + * @inode: inode to put
> + *
> + * Returns true if the reference was dropped, false if this was the last
> + * reference and the caller must arrange for final iput() in a safe context.
> + */
> +static inline bool __must_check iput_if_not_last(struct inode *inode)
> +{
> +	return atomic_add_unless(&inode->i_count, -1, 1);
> +}
> +

This still could assert on the count, for example:
	VFS_BUG_ON_INODE(inode_state_read_once(inode) & (I_FREEING | I_CLEAR), inode);
	VFS_BUG_ON_INODE(atomic_read(&inode->i_count) < 1, inode);

>  int inode_update_time(struct inode *inode, enum fs_update_time type,
>  		unsigned int flags);
>  int generic_update_time(struct inode *inode, enum fs_update_time type,
> -- 
> 2.43.0
>