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Date:   Fri, 1 May 2020 14:17:03 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH 08/24] rcu/tree: Use static initializer for krc.lock

On Tue, Apr 28, 2020 at 10:58:47PM +0200, Uladzislau Rezki (Sony) wrote:
> From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> 
> The per-CPU variable is initialized at runtime in
> kfree_rcu_batch_init(). This function is invoked before
> 'rcu_scheduler_active' is set to 'RCU_SCHEDULER_RUNNING'.
> After the initialisation, '->initialized' is to true.
> 
> The raw_spin_lock is only acquired if '->initialized' is
> set to true. The worqueue item is only used if 'rcu_scheduler_active'
> set to RCU_SCHEDULER_RUNNING which happens after initialisation.
> 
> Use a static initializer for krc.lock and remove the runtime
> initialisation of the lock. Since the lock can now be always
> acquired, remove the '->initialized' check.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  kernel/rcu/tree.c | 15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index bc6c2bc8fa32..89e9ca3f4e3e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2892,7 +2892,7 @@ struct kfree_rcu_cpu_work {
>   * @lock: Synchronize access to this structure
>   * @monitor_work: Promote @head to @head_free after KFREE_DRAIN_JIFFIES
>   * @monitor_todo: Tracks whether a @monitor_work delayed work is pending
> - * @initialized: The @lock and @rcu_work fields have been initialized
> + * @initialized: The @rcu_work fields have been initialized
>   *
>   * This is a per-CPU structure.  The reason that it is not included in
>   * the rcu_data structure is to permit this code to be extracted from
> @@ -2912,7 +2912,9 @@ struct kfree_rcu_cpu {
>  	int count;
>  };
>  
> -static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc);
> +static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc) = {
> +	.lock = __RAW_SPIN_LOCK_UNLOCKED(krc.lock),
> +};
>  
>  static __always_inline void
>  debug_rcu_bhead_unqueue(struct kfree_rcu_bulk_data *bhead)
> @@ -2930,10 +2932,9 @@ krc_this_cpu_lock(unsigned long *flags)
>  {
>  	struct kfree_rcu_cpu *krcp;
>  
> -	local_irq_save(*flags);	// For safely calling this_cpu_ptr().
> +	local_irq_save(*flags);	/* For safely calling this_cpu_ptr(). */

And here as well.  ;-)

							Thanx, Paul

>  	krcp = this_cpu_ptr(&krc);
> -	if (likely(krcp->initialized))
> -		raw_spin_lock(&krcp->lock);
> +	raw_spin_lock(&krcp->lock);
>  
>  	return krcp;
>  }
> @@ -2941,8 +2942,7 @@ krc_this_cpu_lock(unsigned long *flags)
>  static inline void
>  krc_this_cpu_unlock(struct kfree_rcu_cpu *krcp, unsigned long flags)
>  {
> -	if (likely(krcp->initialized))
> -		raw_spin_unlock(&krcp->lock);
> +	raw_spin_unlock(&krcp->lock);
>  	local_irq_restore(flags);
>  }
>  
> @@ -4168,7 +4168,6 @@ static void __init kfree_rcu_batch_init(void)
>  	for_each_possible_cpu(cpu) {
>  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
>  
> -		raw_spin_lock_init(&krcp->lock);
>  		for (i = 0; i < KFREE_N_BATCHES; i++) {
>  			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
>  			krcp->krw_arr[i].krcp = krcp;
> -- 
> 2.20.1
> 

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