netdev - Re: [PATCH net-next 3/3] packet: use percpu mmap tx frame pending refcount

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Message-ID: <CAHA+R7NnaxXgOGW=jTEGkeMByO7wXVr9bGR-J9rLzwL4ne52bA@mail.gmail.com>
Date:	Sun, 12 Jan 2014 21:51:48 -0800
From:	Cong Wang <cwang@...pensource.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 3/3] packet: use percpu mmap tx frame pending refcount

On Sun, Jan 12, 2014 at 8:22 AM, Daniel Borkmann <dborkman@...hat.com> wrote:
> +static void packet_inc_pending(struct packet_ring_buffer *rb)
> +{
> +       this_cpu_inc(*rb->pending_refcnt);
> +}
> +
> +static void packet_dec_pending(struct packet_ring_buffer *rb)
> +{
> +       this_cpu_dec(*rb->pending_refcnt);
> +}
> +
> +static int packet_read_pending(const struct packet_ring_buffer *rb)
> +{
> +       int i, refcnt = 0;
> +
> +       /* We don't use pending refcount in rx_ring. */
> +       if (rb->pending_refcnt == NULL)
> +               return 0;
> +
> +       for_each_possible_cpu(i)
> +               refcnt += *per_cpu_ptr(rb->pending_refcnt, i);
> +
> +       return refcnt;
> +}

How is this supposed to work? Since there is no lock,
you can't read accurate refcnt. Take a look at lib/percpu_counter.c.

I guess for some reason you don't care the accuracy?
Then at least you need to comment in the code.

Thanks.
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