Re: [PATCH v3] eventpoll: Fix priority inversion problem

[John Ogness <john.ogness@linutronix.de>]

On 2025-05-27, Nam Cao <namcao@linutronix.de> wrote:
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index d4dbffdedd08e..a97a771a459c9 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -137,13 +137,7 @@ struct epitem {
>  	};
>  
>  	/* List header used to link this structure to the eventpoll ready list */
> -	struct list_head rdllink;
> -
> -	/*
> -	 * Works together "struct eventpoll"->ovflist in keeping the
> -	 * single linked chain of items.
> -	 */
> -	struct epitem *next;
> +	struct llist_node rdllink;
>  
>  	/* The file descriptor information this item refers to */
>  	struct epoll_filefd ffd;
> @@ -191,22 +185,15 @@ struct eventpoll {
>  	/* Wait queue used by file->poll() */
>  	wait_queue_head_t poll_wait;
>  
> -	/* List of ready file descriptors */
> -	struct list_head rdllist;
> -
> -	/* Lock which protects rdllist and ovflist */
> -	rwlock_t lock;
> +	/*
> +	 * List of ready file descriptors. Adding to this list is lockless. Items can be removed
> +	 * only with eventpoll::mtx
> +	 */
> +	struct llist_head rdllist;
>  
>  	/* RB tree root used to store monitored fd structs */
>  	struct rb_root_cached rbr;
>  
> -	/*
> -	 * This is a single linked list that chains all the "struct epitem" that
> -	 * happened while transferring ready events to userspace w/out
> -	 * holding ->lock.
> -	 */
> -	struct epitem *ovflist;
> -
>  	/* wakeup_source used when ep_send_events or __ep_eventpoll_poll is running */
>  	struct wakeup_source *ws;
>  
> @@ -361,10 +348,14 @@ static inline int ep_cmp_ffd(struct epoll_filefd *p1,
>  	        (p1->file < p2->file ? -1 : p1->fd - p2->fd));
>  }
>  
> -/* Tells us if the item is currently linked */
> -static inline int ep_is_linked(struct epitem *epi)
> +/*
> + * Add the item to its container eventpoll's rdllist; do nothing if the item is already on rdllist.
> + */
> +static void epitem_ready(struct epitem *epi)
>  {
> -	return !list_empty(&epi->rdllink);
> +	if (&epi->rdllink == cmpxchg(&epi->rdllink.next, &epi->rdllink, NULL))

Perhaps:

	if (try_cmpxchg(&epi->rdllink.next, &epi->rdllink, NULL))

> +		llist_add(&epi->rdllink, &epi->ep->rdllist);
> +
>  }
>  
>  static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_entry_t *p)

John Ogness