linux-ext4 - Re: [PATCH v2 22/54] fs: convert i_count to refcount

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Message-ID: <20250828-bergkette-umtriebe-921d8f3bdf4e@brauner>
Date: Thu, 28 Aug 2025 14:00:03 +0200
From: Christian Brauner <brauner@...nel.org>
To: Josef Bacik <josef@...icpanda.com>
Cc: linux-fsdevel@...r.kernel.org, linux-btrfs@...r.kernel.org, 
	kernel-team@...com, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, 
	viro@...iv.linux.org.uk, amir73il@...il.com
Subject: Re: [PATCH v2 22/54] fs: convert i_count to refcount_t

On Tue, Aug 26, 2025 at 11:39:22AM -0400, Josef Bacik wrote:
> Now that we do not allow i_count to drop to 0 and be used we can convert
> it to a refcount_t and benefit from the protections those helpers add.
> 
> Signed-off-by: Josef Bacik <josef@...icpanda.com>
> ---
>  fs/btrfs/inode.c   | 2 +-
>  fs/inode.c         | 9 +++++----
>  include/linux/fs.h | 6 +++---
>  3 files changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index e16df38e0eef..eb9496342346 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -3418,7 +3418,7 @@ void btrfs_add_delayed_iput(struct btrfs_inode *inode)
>  	struct btrfs_fs_info *fs_info = inode->root->fs_info;
>  	unsigned long flags;
>  
> -	if (atomic_add_unless(&inode->vfs_inode.i_count, -1, 1)) {
> +	if (refcount_dec_not_one(&inode->vfs_inode.i_count)) {

Now this is the only place outside core VFS where we open-access
i_count. Add a helper and reuse it iput() as well? icount_maybe_dec()?
icount_dec_not_one()?

>  		iobj_put(&inode->vfs_inode);
>  		return;
>  	}
> diff --git a/fs/inode.c b/fs/inode.c
> index 1992db5cd70a..0be1c137bf1e 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -236,7 +236,7 @@ int inode_init_always_gfp(struct super_block *sb, struct inode *inode, gfp_t gfp
>  	inode->i_state = 0;
>  	atomic64_set(&inode->i_sequence, 0);
>  	refcount_set(&inode->i_obj_count, 1);
> -	atomic_set(&inode->i_count, 1);
> +	refcount_set(&inode->i_count, 1);
>  	inode->i_op = &empty_iops;
>  	inode->i_fop = &no_open_fops;
>  	inode->i_ino = 0;
> @@ -545,7 +545,8 @@ static void init_once(void *foo)
>  void ihold(struct inode *inode)
>  {
>  	iobj_get(inode);
> -	WARN_ON(atomic_inc_return(&inode->i_count) < 2);
> +	refcount_inc(&inode->i_count);
> +	WARN_ON(icount_read(inode) < 2);
>  }
>  EXPORT_SYMBOL(ihold);
>  
> @@ -2011,7 +2012,7 @@ static void __iput(struct inode *inode, bool skip_lru)
>  		return;
>  	BUG_ON(inode->i_state & I_CLEAR);
>  
> -	if (atomic_add_unless(&inode->i_count, -1, 1)) {
> +	if (refcount_dec_not_one(&inode->i_count)) {
>  		iobj_put(inode);
>  		return;
>  	}
> @@ -2031,7 +2032,7 @@ static void __iput(struct inode *inode, bool skip_lru)
>  	 */
>  	drop = maybe_add_lru(inode, skip_lru);
>  
> -	if (atomic_dec_and_test(&inode->i_count)) {
> +	if (refcount_dec_and_test(&inode->i_count)) {
>  		/* iput_final() drops i_lock */
>  		iput_final(inode, drop);
>  	} else {
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 999ffea2aac1..fc23e37ca250 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -875,7 +875,7 @@ struct inode {
>  	};
>  	atomic64_t		i_version;
>  	atomic64_t		i_sequence; /* see futex */
> -	atomic_t		i_count;
> +	refcount_t		i_count;
>  	atomic_t		i_dio_count;
>  	atomic_t		i_writecount;
>  #if defined(CONFIG_IMA) || defined(CONFIG_FILE_LOCKING)
> @@ -3399,12 +3399,12 @@ static inline unsigned int iobj_count_read(const struct inode *inode)
>  static inline void __iget(struct inode *inode)
>  {
>  	iobj_get(inode);
> -	atomic_inc(&inode->i_count);
> +	refcount_inc(&inode->i_count);
>  }
>  
>  static inline int icount_read(const struct inode *inode)
>  {
> -	return atomic_read(&inode->i_count);
> +	return refcount_read(&inode->i_count);
>  }
>  
>  extern void iget_failed(struct inode *);
> -- 
> 2.49.0
>