linux-kernel - Re: [PATCH v2] rcu/segcblist: Add debug checks for segment lengths

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Date:   Wed, 18 Nov 2020 12:13:35 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2] rcu/segcblist: Add debug checks for segment lengths

On Wed, Nov 18, 2020 at 11:15:41AM -0500, Joel Fernandes (Google) wrote:
> After rcu_do_batch(), add a check for whether the seglen counts went to
> zero if the list was indeed empty.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Queued for testing and further review, thank you!

							Thanx, Paul

> ---
> v1->v2: Added more debug checks.
> 
>  kernel/rcu/rcu_segcblist.c | 12 ++++++++++++
>  kernel/rcu/rcu_segcblist.h |  3 +++
>  kernel/rcu/tree.c          |  2 ++
>  3 files changed, 17 insertions(+)
> 
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 5059b6102afe..6e98bb3804f0 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -94,6 +94,18 @@ static long rcu_segcblist_get_seglen(struct rcu_segcblist *rsclp, int seg)
>  	return READ_ONCE(rsclp->seglen[seg]);
>  }
>  
> +/* Return number of callbacks in segmented callback list by totalling seglen. */
> +long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp)
> +{
> +	long len = 0;
> +	int i;
> +
> +	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
> +		len += rcu_segcblist_get_seglen(rsclp, i);
> +
> +	return len;
> +}
> +
>  /* Set the length of a segment of the rcu_segcblist structure. */
>  static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long v)
>  {
> diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> index cd35c9faaf51..46a42d77f7e1 100644
> --- a/kernel/rcu/rcu_segcblist.h
> +++ b/kernel/rcu/rcu_segcblist.h
> @@ -15,6 +15,9 @@ static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
>  	return READ_ONCE(rclp->len);
>  }
>  
> +/* Return number of callbacks in segmented callback list by totalling seglen. */
> +long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp);
> +
>  void rcu_cblist_init(struct rcu_cblist *rclp);
>  void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp);
>  void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f5b61e10f1de..91e35b521e51 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2553,6 +2553,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
>  	WARN_ON_ONCE(count == 0 && !rcu_segcblist_empty(&rdp->cblist));
>  	WARN_ON_ONCE(!IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
>  		     count != 0 && rcu_segcblist_empty(&rdp->cblist));
> +	WARN_ON_ONCE(count == 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) != 0);
> +	WARN_ON_ONCE(count != 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);
>  
>  	rcu_nocb_unlock_irqrestore(rdp, flags);
>  
> -- 
> 2.29.2.299.gdc1121823c-goog