Re: [PATCH v3 3/3] rcu: Use _full() API to debug synchronize_rcu()

[Boqun Feng <boqun.feng@gmail.com>]

Hi Ulad,

On Tue, Feb 25, 2025 at 12:00:20PM +0100, Uladzislau Rezki (Sony) wrote:
> Switch for using of get_state_synchronize_rcu_full() and
> poll_state_synchronize_rcu_full() pair for debug a normal
> synchronize_rcu() call.
> 
> Just using "not" full APIs to identify if a grace period
> is passed or not might lead to a false kernel splat.
> 

Could you provide detailed explanation on this? I.e. why is _full() is
needed? I find the current commit message is a bit vague.

Regards,
Boqun

> Link: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/
> Fixes: 988f569ae041 ("rcu: Reduce synchronize_rcu() latency")
> Reported-by: cheung wall <zzqq0103.hey@gmail.com>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  include/linux/rcupdate_wait.h | 3 +++
>  kernel/rcu/tree.c             | 8 +++-----
>  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
> index f9bed3d3f78d..4c92d4291cce 100644
> --- a/include/linux/rcupdate_wait.h
> +++ b/include/linux/rcupdate_wait.h
> @@ -16,6 +16,9 @@
>  struct rcu_synchronize {
>  	struct rcu_head head;
>  	struct completion completion;
> +
> +	/* This is for debugging. */
> +	struct rcu_gp_oldstate oldstate;
>  };
>  void wakeme_after_rcu(struct rcu_head *head);
>  
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8625f616c65a..48384fa2eaeb 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1632,12 +1632,10 @@ static void rcu_sr_normal_complete(struct llist_node *node)
>  {
>  	struct rcu_synchronize *rs = container_of(
>  		(struct rcu_head *) node, struct rcu_synchronize, head);
> -	unsigned long oldstate = (unsigned long) rs->head.func;
>  
>  	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
> -		!poll_state_synchronize_rcu(oldstate),
> -		"A full grace period is not passed yet: %lu",
> -		rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
> +		!poll_state_synchronize_rcu_full(&rs->oldstate),
> +		"A full grace period is not passed yet!\n");
>  
>  	/* Finally. */
>  	complete(&rs->completion);
> @@ -3247,7 +3245,7 @@ static void synchronize_rcu_normal(void)
>  	 * snapshot before adding a request.
>  	 */
>  	if (IS_ENABLED(CONFIG_PROVE_RCU))
> -		rs.head.func = (void *) get_state_synchronize_rcu();
> +		get_state_synchronize_rcu_full(&rs.oldstate);
>  
>  	rcu_sr_normal_add_req(&rs);
>  
> -- 
> 2.39.5
>