Re: [PATCH v10 1/5] fs: add iput_if_not_last() helper

[Jan Kara]

On Thu 25-06-26 23:29:37, 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@xxxxxxxxxxxxx>
> Suggested-by: Jan Kara <jack@xxxxxxx>

Yes, I think this is a sensible addition to the inode refcounting API which
will allow filesystems to save the hassle of iput offloading in the common
case. Just one nit below, otherwise feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 6da44573ce45..4916a9d54347 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2418,6 +2418,19 @@ 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 iput_if_not_last(struct inode *inode)

I'd add __must_check to the declaration.

								Honza

> +{
> +	return atomic_add_unless(&inode->i_count, -1, 1);
> +}
> +
>  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
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR