Re: [PATCH v5 1/7] eventfs: Remove "is_freed" union with rcu head

Masami Hiramatsu
2023-11-01T12:20:09Z

On Tue, 31 Oct 2023 18:33:27 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
>
> The eventfs_inode->is_freed was a union with the rcu_head with the
> assumption that when it was on the srcu list the head would contain a
> pointer which would make "is_freed" true. But that was a wrong assumption
> as the rcu head is a single link list where the last element is NULL.
>
> Instead, split the nr_entries integer so that "is_freed" is one bit and
> the nr_entries is the next 31 bits. As there shouldn't be more than 10
> (currently there's at most 5 to 7 depending on the config), this should
> not be a problem.

Yeah, even 16 bit nr_entries is enough.. (maybe the biggest user is
syscall event group)

Looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you,

> Cc: stable@vger.kernel.org
> Fixes: 63940449555e7 ("eventfs: Implement eventfs lookup, read, open functions")
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> fs/tracefs/event_inode.c | 2 ++
> fs/tracefs/internal.h | 6 +++---
> 2 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
> index 754885dfe71c..2c2c75b2ad73 100644
> --- a/fs/tracefs/event_inode.c
> +++ b/fs/tracefs/event_inode.c
> @@ -824,6 +824,8 @@ static void eventfs_remove_rec(struct eventfs_inode *ei, struct list_head *head,
> eventfs_remove_rec(ei_child, head, level + 1);
> }
>
> + ei->is_freed = 1;
> +
> list_del_rcu(&ei->list);
> list_add_tail(&ei->del_list, head);
> }
> diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
> index 64fde9490f52..c7d88aaa949f 100644
> --- a/fs/tracefs/internal.h
> +++ b/fs/tracefs/internal.h
> @@ -23,6 +23,7 @@ struct tracefs_inode {
> * @d_parent: pointer to the parent's dentry
> * @d_children: The array of dentries to represent the files when created
> * @data: The private data to pass to the callbacks
> + * @is_freed: Flag set if the eventfs is on its way to be freed
> * @nr_entries: The number of items in @entries
> */
> struct eventfs_inode {
> @@ -38,14 +39,13 @@ struct eventfs_inode {
> * Union - used for deletion
> * @del_list: list of eventfs_inode to delete
> * @rcu: eventfs_inode to delete in RCU
> - * @is_freed: node is freed if one of the above is set
> */
> union {
> struct list_head del_list;
> struct rcu_head rcu;
> - unsigned long is_freed;
> };
> - int nr_entries;
> + unsigned int is_freed:1;
> + unsigned int nr_entries:31;
> };
>
> static inline struct tracefs_inode *get_tracefs(const struct inode *inode)
> --
> 2.42.0

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>