Re: [Patch v2] doc/RCU/listRCU: refine example code for eliminating stale data

[Alan Huang <mmpgouride@gmail.com>]

On Feb 18, 2025, at 08:50, Wei Yang <richard.weiyang@gmail.com> wrote:
> 
> This patch adjust the example code with following two purpose:
> 
>  * reduce the confusion on not releasing e->lock
>  * emphasize e is valid and not stale with e->lock held
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> CC: Boqun Feng <boqun.feng@gmail.com>
> CC: Alan Huang <mmpgouride@gmail.com>
> 
> ---
> v2:
>  * add the missing parameter *key
>  * make function return struct audit_entry
> ---
> Documentation/RCU/listRCU.rst | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/RCU/listRCU.rst b/Documentation/RCU/listRCU.rst
> index ed5c9d8c9afe..d8bb98623c12 100644
> --- a/Documentation/RCU/listRCU.rst
> +++ b/Documentation/RCU/listRCU.rst
> @@ -334,7 +334,7 @@ If the system-call audit module were to ever need to reject stale data, one way
> to accomplish this would be to add a ``deleted`` flag and a ``lock`` spinlock to the
> ``audit_entry`` structure, and modify audit_filter_task() as follows::
> 
> - static enum audit_state audit_filter_task(struct task_struct *tsk)
> + static struct audit_entry *audit_filter_task(struct task_struct *tsk, char **key)
> {
> struct audit_entry *e;
> enum audit_state   state;
> @@ -346,16 +346,18 @@ to accomplish this would be to add a ``deleted`` flag and a ``lock`` spinlock to
> if (e->deleted) {
> spin_unlock(&e->lock);
> rcu_read_unlock();
> - return AUDIT_BUILD_CONTEXT;
> + return NULL;
> }
> rcu_read_unlock();
> if (state == AUDIT_STATE_RECORD)
> *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
> - return state;
> + /* As long as e->lock is held, e is valid and
> + * its value is not stale */
> + return e;
> }
> }
> rcu_read_unlock();
> - return AUDIT_BUILD_CONTEXT;
> + return NULL;
> }
> 
> The ``audit_del_rule()`` function would need to set the ``deleted`` flag under the
> -- 
> 2.34.1
> 

I think it’s good enough to illustrate the intention here:

Reviewed-by: Alan Huang <mmpgouride@gmail.com>

Boqun, there is an unreviewed doc patch[1] that fixes the section

 “Using RCU hlist_nulls to protect list and objects”

[1] https://lore.kernel.org/rcu/20240326124431.77430-1-mmpgouride@gmail.com/

: )