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

[Jan Kara <jack@suse.cz>]

On Tue 30-06-26 11:05:50, Mateusz Guzik wrote:
> 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);

Yes, it makes sense to be consistent with other helpers. Thanks for the
idea.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR