netdev - Re: [PATCH net-next] tcp: use 2-arg optimal variant of kfree

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Date:   Fri, 2 Dec 2022 16:03:25 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        eric.dumazet@...il.com, Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH net-next] tcp: use 2-arg optimal variant of kfree_rcu()

On Fri, Dec 02, 2022 at 11:49:59PM +0000, Joel Fernandes wrote:
> On Fri, Dec 02, 2022 at 05:28:47AM +0000, Eric Dumazet wrote:
> > kfree_rcu(1-arg) should be avoided as much as possible,
> > since this is only possible from sleepable contexts,
> > and incurr extra rcu barriers.
> > 
> > I wish the 1-arg variant of kfree_rcu() would
> > get a distinct name, like kfree_rcu_slow()
> > to avoid it being abused.
> 
> Hi Eric,
> Nice to see your patch.
> 
> Paul, all, regarding Eric's concern, would the following work to warn of
> users? Credit to Paul/others for discussing the idea on another thread. One
> thing to note here is, this debugging will only be in effect on preemptible
> kernels, but should still help catch issues hopefully.

Mightn't there be some places where someone needs to invoke
single-argument kfree_rcu() in a preemptible context, for example,
due to the RCU-protected structure being very small and very numerous?

> The other idea Paul mentioned is to introduce a new dedicated API for 1-arg
> sleepable cases. My concern with that was that, that being effective depends
> on the user using the right API in the first place.

Actually, Eric's idea from above.  ;-)

							Thanx, Paul

> I did not test it yet, but wanted to discuss a bit first.
> 
> Cheers,
> 
> - Joel
> 
> ---8<-----------------------
> 
> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> index 9bc025aa79a3..112d230279ea 100644
> --- a/include/linux/rcutiny.h
> +++ b/include/linux/rcutiny.h
> @@ -106,6 +106,11 @@ static inline void __kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  	}
>  
>  	// kvfree_rcu(one_arg) call.
> +	if (IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible() && !head) {
> +		WARN_ONCE(1, "%s(): Please provide an rcu_head in preemptible"
> +			  " contexts to avoid long waits!\n", __func__);
> +	}
> +
>  	might_sleep();
>  	synchronize_rcu();
>  	kvfree((void *) func);
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0ca21ac0f064..b29df1305a2e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3324,6 +3324,11 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
>  		 * only. For other places please embed an rcu_head to
>  		 * your data.
>  		 */
> +		if (IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible() && !head) {
> +			WARN_ONCE(1, "%s(): Please provide an rcu_head in preemptible"
> +				  " contexts to avoid long waits!\n", __func__);
> +		}
> +
>  		might_sleep();
>  		ptr = (unsigned long *) func;
>  	}