Re: [PATCH v2] ocfs2: kill osb->system_file_mutex lock

[Heming Zhao]

On Mon, May 18, 2026 at 01:23:40PM +0900, Tetsuo Handa wrote:
> Commit 43b10a20372d ("ocfs2: avoid system inode ref confusion by adding
> mutex lock") tried to avoid a refcount leak caused by allowing multiple
> threads to call igrab(inode). But addition of osb->system_file_mutex made
> locking dependency complicated and is causing lockdep to warn about
> possibility of AB-BA deadlock.
> 
> Since _ocfs2_get_system_file_inode() returns the same inode for the same
> input arguments, we don't need to serialize _ocfs2_get_system_file_inode().
> What we need to make sure is that igrab(inode) is called for only once().
> Therefore, replace osb->system_file_mutex with cmpxchg()-based locking.
> 
> Fixes: 43b10a20372d ("ocfs2: avoid system inode ref confusion by adding mutex lock")
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>

LGTM. Thanks for the patch.
Reviewed-by: Heming Zhao <heming.zhao@xxxxxxxx>
> ---
> Changes in v2:
>   Updated patch description.
> 
>  fs/ocfs2/ocfs2.h   | 2 --
>  fs/ocfs2/super.c   | 2 --
>  fs/ocfs2/sysfile.c | 9 +++------
>  3 files changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/ocfs2/ocfs2.h b/fs/ocfs2/ocfs2.h
> index 7b50e03dfa66..62cad6522c7a 100644
> --- a/fs/ocfs2/ocfs2.h
> +++ b/fs/ocfs2/ocfs2.h
> @@ -494,8 +494,6 @@ struct ocfs2_super
>  	struct rb_root	osb_rf_lock_tree;
>  	struct ocfs2_refcount_tree *osb_ref_tree_lru;
>  
> -	struct mutex system_file_mutex;
> -
>  	/*
>  	 * OCFS2 needs to schedule several different types of work which
>  	 * require cluster locking, disk I/O, recovery waits, etc. Since these
> diff --git a/fs/ocfs2/super.c b/fs/ocfs2/super.c
> index b875f01c9756..6dd45c2153f8 100644
> --- a/fs/ocfs2/super.c
> +++ b/fs/ocfs2/super.c
> @@ -1997,8 +1997,6 @@ static int ocfs2_initialize_super(struct super_block *sb,
>  	spin_lock_init(&osb->osb_xattr_lock);
>  	ocfs2_init_steal_slots(osb);
>  
> -	mutex_init(&osb->system_file_mutex);
> -
>  	atomic_set(&osb->alloc_stats.moves, 0);
>  	atomic_set(&osb->alloc_stats.local_data, 0);
>  	atomic_set(&osb->alloc_stats.bitmap_data, 0);
> diff --git a/fs/ocfs2/sysfile.c b/fs/ocfs2/sysfile.c
> index d53a6cc866be..67e492f4b828 100644
> --- a/fs/ocfs2/sysfile.c
> +++ b/fs/ocfs2/sysfile.c
> @@ -98,11 +98,9 @@ struct inode *ocfs2_get_system_file_inode(struct ocfs2_super *osb,
>  	} else
>  		arr = get_local_system_inode(osb, type, slot);
>  
> -	mutex_lock(&osb->system_file_mutex);
>  	if (arr && ((inode = *arr) != NULL)) {
>  		/* get a ref in addition to the array ref */
>  		inode = igrab(inode);
> -		mutex_unlock(&osb->system_file_mutex);
>  		BUG_ON(!inode);
>  
>  		return inode;
> @@ -112,11 +110,10 @@ struct inode *ocfs2_get_system_file_inode(struct ocfs2_super *osb,
>  	inode = _ocfs2_get_system_file_inode(osb, type, slot);
>  
>  	/* add one more if putting into array for first time */
> -	if (arr && inode) {
> -		*arr = igrab(inode);
> -		BUG_ON(!*arr);
> +	if (inode && arr && !*arr && !cmpxchg(&(*arr), NULL, inode)) {
> +		inode = igrab(inode);
> +		BUG_ON(!inode);
>  	}
> -	mutex_unlock(&osb->system_file_mutex);
>  	return inode;
>  }
>  
> -- 
> 2.54.0
> 
>