linux-kernel - Re: [PATCH 4/4] rcu: Use _full() API to debug synchronize

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Message-ID: <ea7d8677-f81e-4590-9716-23ea41597873@paulmck-laptop>
Date: Thu, 23 Jan 2025 13:52:57 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>, RCU <rcu@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Cheung Wall <zzqq0103.hey@...il.com>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 4/4] rcu: Use _full() API to debug synchronize_rcu()

On Thu, Jan 23, 2025 at 07:58:28PM +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.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  include/linux/rcupdate_wait.h | 4 ++++
>  kernel/rcu/tree.c             | 8 +++-----
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
> index f9bed3d3f78d..a16fc2a9a7d7 100644
> --- a/include/linux/rcupdate_wait.h
> +++ b/include/linux/rcupdate_wait.h
> @@ -16,6 +16,10 @@
>  struct rcu_synchronize {
>  	struct rcu_head head;
>  	struct completion completion;
> +#ifdef CONFIG_PROVE_RCU
> +	/* This is for testing. */
> +	struct rcu_gp_oldstate oldstate;
> +#endif
>  };
>  void wakeme_after_rcu(struct rcu_head *head);
>  
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 2795d6b5109c..0ae90089ef09 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1612,12 +1612,10 @@ 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");

Looks good, but why not also continue printing out the required
grace-period sequence number?  Yes, there would need to be helper
sprintf()-style functions to paper over the difference between Tiny RCU
and Tree RCU.  ;-)

							Thanx, Paul

>  	/* Finally. */
>  	complete(&rs->completion);
> @@ -3214,7 +3212,7 @@ 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
>