linux-kernel - Re: [PATCH v5 07/15] locking/lockdep: Free lock classes that are no longer in use

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Message-ID: <1547146287.83374.49.camel@acm.org>
Date:   Thu, 10 Jan 2019 10:51:27 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, tj@...nel.org, longman@...hat.com,
        johannes.berg@...el.com, linux-kernel@...r.kernel.org,
        Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH v5 07/15] locking/lockdep: Free lock classes that are no
 longer in use

On Thu, 2019-01-10 at 16:24 +0100, Peter Zijlstra wrote:
>  /*
>   * A data structure for delayed freeing of data structures that may be
> - * accessed by RCU readers at the time these were freed. The size of the array
> - * is a compromise between minimizing the amount of memory used by this array
> - * and minimizing the number of wait_event() calls by get_pending_free_lock().
> + * accessed by RCU readers at the time these were freed.
>   */
>  static struct pending_free {
> -	struct list_head zapped_classes;
>  	struct rcu_head	 rcu_head;
> +	int		 index;
>  	int		 pending;
> -} pending_free[2];
> -static DECLARE_WAIT_QUEUE_HEAD(rcu_cb);
> +	struct list_head zapped[2];
> +} pending_free;

Hi Peter,

If the zapped[] array only has two elements there is no guarantee that an
element will be free when zap_class() is called. I think we need at least
num_online_cpus() elements to guarantee that at least one element is free
when zap_class() is called. So removing the wait loop from
get_pending_free_lock() seems wrong to me. Have you tried to run a workload
that keeps all CPUs busy and that triggers get_pending_free_lock()
frequently?

Thanks,

Bart.