Re: [PATCH] s390/pkey: Remove synchronize_rcu from pkey_handler_register

[Harald Freudenberger <freude@linux.ibm.com>]

On 2026-03-13 06:23, lirongqing wrote:
> From: Li RongQing <lirongqing@baidu.com>
> 
> The synchronize_rcu() call after adding a handler to the handler_list
> is redundant because RCU readers will either see the old list or the
> new list. Removing this synchronization point reduces the blocking
> time during handler registration.
> 
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>  drivers/s390/crypto/pkey_base.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/s390/crypto/pkey_base.c 
> b/drivers/s390/crypto/pkey_base.c
> index d60cd98..c2e29d6 100644
> --- a/drivers/s390/crypto/pkey_base.c
> +++ b/drivers/s390/crypto/pkey_base.c
> @@ -60,7 +60,6 @@ int pkey_handler_register(struct pkey_handler 
> *handler)
> 
>  	list_add_rcu(&handler->list, &handler_list);
>  	spin_unlock(&handler_list_write_lock);
> -	synchronize_rcu();
> 
>  	module_put(handler->module);

After some deep dive into RCU, and some code review with Holger we
decided to leave this statement in the code: This is the fast path to
push the info about the updated list to the other cpus. If removed, the
other cpus may get the updated list when the RCU context is synched. As
this code is in general not performance critical and the list update
more or less only occurs at the early time in system startup the focus
is on concurrency versus performance. Maybe I'll add a comment onto
this code line...

Thanks Li RongQing for your patch and suggestions.

Harald Freudenberger