Re: [PATCH v4 3/3] rcu: Use _full() API to debug synchronize_rcu()

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

Hi Ulad,

I put these three patches into next (and misc.2025.02.27a) for some
testing, hopefully it all goes well and they can make it v6.15.

A few tag changed below:

On Thu, Feb 27, 2025 at 02:16:13PM +0100, Uladzislau Rezki (Sony) wrote:
> Switch for using of get_state_synchronize_rcu_full() and
> poll_state_synchronize_rcu_full() pair to 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-positive kernel splat.
> 
> It can happen, because get_state_synchronize_rcu() compresses
> both normal and expedited states into one single unsigned long
> value, so a poll_state_synchronize_rcu() can miss GP-completion
> when synchronize_rcu()/synchronize_rcu_expedited() concurrently
> run.
> 
> To address this, switch to poll_state_synchronize_rcu_full() and
> get_state_synchronize_rcu_full() APIs, which use separate variables
> for expedited and normal states.
> 
> Link: https://lore.kernel.org/lkml/Z5ikQeVmVdsWQrdD@pc636/T/

I switch this into "Closes:" per checkpatch.

> Fixes: 988f569ae041 ("rcu: Reduce synchronize_rcu() latency")
> Reported-by: cheung wall <zzqq0103.hey@gmail.com>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

You seem to forget add Paul's Reviewed-by, so I add it in rcu/next.
Would you or Paul double-check the Reviewed-by should be here?

Regards,
Boqun

> ---
>  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
>