linux-kernel - Re: [PATCH 21/24] rcu/tiny: move kvfree_call

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Date:   Fri, 1 May 2020 16:03:59 -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>
Subject: Re: [PATCH 21/24] rcu/tiny: move kvfree_call_rcu() out of header

On Tue, Apr 28, 2020 at 10:59:00PM +0200, Uladzislau Rezki (Sony) wrote:
> Move inlined kvfree_call_rcu() function out of the
> header file. This step is a preparation for head-less
> support.
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  include/linux/rcutiny.h | 6 +-----
>  kernel/rcu/tiny.c       | 6 ++++++
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> index 0c6315c4a0fe..7eb66909ae1b 100644
> --- a/include/linux/rcutiny.h
> +++ b/include/linux/rcutiny.h
> @@ -34,11 +34,7 @@ static inline void synchronize_rcu_expedited(void)
>  	synchronize_rcu();
>  }
>  
> -static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> -{
> -	call_rcu(head, func);
> -}
> -
> +void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
>  void rcu_qs(void);
>  
>  static inline void rcu_softirq_qs(void)
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index aa897c3f2e92..508c82faa45c 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -177,6 +177,12 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
>  }
>  EXPORT_SYMBOL_GPL(call_rcu);
>  
> +void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> +{
> +	call_rcu(head, func);
> +}
> +EXPORT_SYMBOL_GPL(kvfree_call_rcu);

This increases the size of Tiny RCU.  Plus in Tiny RCU, the overhead of
synchronize_rcu() is exactly zero.  So why not make the single-argument
kvfree_call_rcu() just unconditionally do synchronize_rcu() followed by
kvfree() or whatever?  That should go just fine into the header file.

							Thanx, Paul

>  void __init rcu_init(void)
>  {
>  	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
> -- 
> 2.20.1
>