netdev - Re: [PATCH net-next v2] rps: selective flow shedding during softnet overflow

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Message-ID: <1366669808.16391.90.camel@edumazet-glaptop>
Date:	Mon, 22 Apr 2013 15:30:08 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	stephen@...workplumber.org
Subject: Re: [PATCH net-next v2] rps: selective flow shedding during softnet
 overflow

On Mon, 2013-04-22 at 16:46 -0400, Willem de Bruijn wrote:

> +		len = sizeof(*cur) + netdev_flow_limit_table_len;
[1]
> +		mutex_lock(&flow_limit_update_mutex);
[2]
> +		for_each_possible_cpu(i) {
> +			sd = &per_cpu(softnet_data, i);
> +			cur = rcu_dereference_protected(sd->flow_limit,
> +				      lockdep_is_held(flow_limit_update_mutex));
> +			if (cur && !cpumask_test_cpu(i, mask)) {
> +				RCU_INIT_POINTER(sd->flow_limit, NULL);
> +				synchronize_rcu();
> +				kfree(cur);
> +			} else if (!cur && cpumask_test_cpu(i, mask)) {
> +				cur = kzalloc(len, GFP_KERNEL);
> +				cur->num_buckets = netdev_flow_limit_table_len;
[3]

Its a bit tricky, but the value of netdev_flow_limit_table_len could
change between [1] and [3]

So you should read its value once, or protect the whole thing using
mutex_lock(&flow_limit_update_mutex) in sysctl code ( and move [1] after
[2])



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