Re: [PATCH v2] eventfs: Use children field for rcu head and add memory barriers

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 7 Aug 2026 20:44:41 -0400
Steven Rostedt <steven@rostedt.org> wrote:

> diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
> index 39c7a34531e8..7d2431f2bb08 100644
> --- a/fs/tracefs/event_inode.c
> +++ b/fs/tracefs/event_inode.c
> @@ -124,7 +124,17 @@ static inline void put_ei(struct eventfs_inode *ei)
>  static inline void free_ei(struct eventfs_inode *ei)
>  {
>  	if (ei) {
> +		/* The ei should have no children if it is being freed. */
> +		WARN_ON_ONCE(!list_empty(&ei->children));
>  		ei->is_freed = 1;
> +		/*
> +		 * The SRCU iteration has a smp_rmb() to make sure it
> +		 * sees a child (that may have already been freed)
> +		 * before it reads is_free. If is_free is set, it must
> +		 * not use the child it acquired from ei->children, as
> +		 * the list may be used for SRCU.
> +		 */
> +		smp_wmb();
>  		put_ei(ei);
>  	}
>  }
> @@ -627,6 +637,20 @@ static int eventfs_iterate(struct file *file, struct dir_context *ctx)
>  	list_for_each_entry_srcu(ei_child, &ei->children, list,
>  				 srcu_read_lock_held(&eventfs_srcu)) {
>  
> +		/*
> +		 * If the ei is being freed, then the ei->children may be
> +		 * being used as the rcu list, which means the next element
> +		 * may be garbage. The ei->is_free is set before switching
> +		 * the ei->children over to ei->rcu. The read memory barrier
> +		 * here makes sure the ei_child is read before is_free is
> +		 * updated.
> +		 *
> +		 * Matches the smp_wmb() in put_ei()

Bah! Now Sashiko is complaining that the wmb is in free_ei() and not
put_ei(). Yes it is correct, but I wish it pointed this out last time
:-p

I'll send a v3.

-- Steve


> +		 */
> +		smp_rmb();
> +		if (ei->is_freed)
> +			return -EINVAL;
> +
>  		if (c > 0) {
>  			c--;
>  			continue;
> diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
> index a4a7f8431aff..c61481d04c8e 100644
> --- a/fs/tracefs/internal.h
> +++ b/fs/tracefs/internal.h
> @@ -46,11 +46,11 @@ struct eventfs_attr {
>   * @ino:	The saved inode number
>   */
>  struct eventfs_inode {
> +	struct list_head	list;
>  	union {
> -		struct list_head	list;
> +		struct list_head	children;
>  		struct rcu_head		rcu;
>  	};
> -	struct list_head		children;
>  	const struct eventfs_entry	*entries;
>  	const char			*name;
>  	struct eventfs_attr		*entry_attrs;