netdev - Re: [PATCH 37/50] netfilter: nf_tables: atomic dump and reset for stateful objects

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Date:   Fri, 09 Dec 2016 06:24:52 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     Paul Gortmaker <paul.gortmaker@...driver.com>,
        netfilter-devel@...r.kernel.org,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        "linux-next@...r.kernel.org" <linux-next@...r.kernel.org>
Subject: Re: [PATCH 37/50] netfilter: nf_tables: atomic dump and reset for
 stateful objects

On Fri, 2016-12-09 at 11:24 +0100, Pablo Neira Ayuso wrote:
> Hi Paul,

Hi Pablo

Given that bytes/packets counters are modified without cmpxchg64()  :

static inline void nft_counter_do_eval(struct nft_counter_percpu_priv *priv,
                                       struct nft_regs *regs,
                                       const struct nft_pktinfo *pkt)
{
        struct nft_counter_percpu *this_cpu;

        local_bh_disable();
        this_cpu = this_cpu_ptr(priv->counter);
        u64_stats_update_begin(&this_cpu->syncp);
        this_cpu->counter.bytes += pkt->skb->len;
        this_cpu->counter.packets++;
        u64_stats_update_end(&this_cpu->syncp);
        local_bh_enable();
}

It means that the cmpxchg64() used to clear the stats is not good enough.

It does not help to make sure stats are properly cleared.

On 64 bit, the ->syncp is not there, so the nft_counter_reset() might
not see that a bytes or packets counter was modified by another cpu.


CPU 1                              CPU 2

LOAD PTR->BYTES into REG_A         old = *counter;
REG_A += skb->len;
                                   cmpxchg64(counter, old, 0);
PTR->BYTES = REG_A

It looks that you want a seqcount, even on 64bit arches,
so that CPU 2 can restart its loop, and more importantly you need
to not accumulate the values you read, because they might be old/invalid.

Another way would be to not use cmpxchg64() at all.
Way to expensive in fast path !

The percpu value would never be modified by an other cpu than the owner.

You need a per cpu seqcount, no need to add a syncp per nft percpu counter.


static DEFINE_PERCPU(seqcount_t, nft_pcpu_seq);

static inline void nft_counter_do_eval(struct nft_counter_percpu_priv *priv,
                                       struct nft_regs *regs,
                                       const struct nft_pktinfo *pkt)
{
        struct nft_counter_percpu *this_cpu;
	seqcount_t *myseq;

        local_bh_disable();
        this_cpu = this_cpu_ptr(priv->counter);
	myseq = this_cpu_ptr(&nft_pcpu_seq);

	write_seqcount_begin(myseq);

        this_cpu->counter.bytes += pkt->skb->len;
        this_cpu->counter.packets++;

	write_seqcount_end(myseq);
	
        local_bh_enable();
}

Thanks !