netdev - Re: [PATCH] pkt_sched: gen_kill

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Date:	Wed, 9 Jun 2010 10:41:00 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Changli Gao <xiaosuo@...il.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH] pkt_sched: gen_kill_estimator() rcu fixes

On Wed, Jun 09, 2010 at 11:56:56AM +0200, Eric Dumazet wrote:
...
> Updated patch follows :
> 
> [PATCH] pkt_sched: gen_kill_estimator() rcu fixes
> 
> gen_kill_estimator() API is a bit misleading, since caller

I'd call it incomplete or simply buggy. Plus a tiny note below.

> should make sure an RCU grace period is respected before
> freeing stats_lock.
> 
> This was partially addressed in commit 5d944c640b4 
> (gen_estimator: deadlock fix), but same problem exist for all
> gen_kill_estimator() users, if lock they use is not already rcu
> protected.
> 
> A code review shows xt_RATEEST.c, act_api.c, act_police.c have this
> problem. Other are ok because they use qdisc lock.
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> ---
...
> diff --git a/net/netfilter/xt_RATEEST.c b/net/netfilter/xt_RATEEST.c
> index 69c01e1..096b18b 100644
> --- a/net/netfilter/xt_RATEEST.c
> +++ b/net/netfilter/xt_RATEEST.c
> @@ -60,13 +60,18 @@ struct xt_rateest *xt_rateest_lookup(const char *name)
>  }
>  EXPORT_SYMBOL_GPL(xt_rateest_lookup);
>  
> +static void xt_rateeset_free_rcu(struct rcu_head *head)
> +{
> +	kfree(container_of(head, struct xt_rateest, rcu));
> +}
> +
>  void xt_rateest_put(struct xt_rateest *est)
>  {
>  	mutex_lock(&xt_rateest_mutex);
>  	if (--est->refcnt == 0) {
>  		hlist_del(&est->list);
>  		gen_kill_estimator(&est->bstats, &est->rstats);
> -		kfree(est);
> +		call_rcu(&est->rcu, xt_rateeset_free_rcu);

Spelling suggestion: xt_rateest_free_rcu?

Since it's only 3 places I'd consider adding little comments,
who needs this call_rcu (like in qdisc_destroy).

Anyway this patch looks OK to me.

Jarek P.
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