netdev - Re: [PATCH net-next v3 2/3] udp: implement memory accounting helpers

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Date:   Wed, 28 Sep 2016 18:42:58 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        James Morris <jmorris@...ei.org>,
        Trond Myklebust <trond.myklebust@...marydata.com>,
        Alexander Duyck <aduyck@...antis.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Tom Herbert <tom@...bertland.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Edward Cree <ecree@...arflare.com>, linux-nfs@...r.kernel.org
Subject: Re: [PATCH net-next v3 2/3] udp: implement memory accounting helpers

On Wed, 2016-09-28 at 12:52 +0200, Paolo Abeni wrote:

> +static void udp_rmem_release(struct sock *sk, int partial)
> +{
> +	struct udp_sock *up = udp_sk(sk);
> +	int fwd, amt;
> +
> +	if (partial && !udp_under_memory_pressure(sk))
> +		return;
> +
> +	/* we can have concurrent release; if we catch any conflict
> +	 * we let only one of them do the work
> +	 */
> +	if (atomic_dec_if_positive(&up->can_reclaim) < 0)
> +		return;
> +
> +	fwd = __udp_forward(up, atomic_read(&sk->sk_rmem_alloc));
> +	if (fwd < SK_MEM_QUANTUM + partial) {
> +		atomic_inc(&up->can_reclaim);
> +		return;
> +	}
> +
> +	amt = (fwd - partial) & ~(SK_MEM_QUANTUM - 1);
> +	atomic_sub(amt, &up->mem_allocated);
> +	atomic_inc(&up->can_reclaim);
> +
> +	__sk_mem_reduce_allocated(sk, amt >> SK_MEM_QUANTUM_SHIFT);
> +	sk->sk_forward_alloc = fwd - amt;
> +}


This is racy... all these atomics make me nervous...