netdev - Re: [PATCH net-next 1/7] r8152: adjust rx

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Message-ID: <CAE4R7bAoTAMMtq0JQK0vmyLqooU88VVuoLkZcFd=1WfxCaqyaw@mail.gmail.com>
Date:	Mon, 26 Jan 2015 01:14:06 -0800
From:	Scott Feldman <sfeldma@...il.com>
To:	Hayes Wang <hayeswang@...ltek.com>
Cc:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	nic_swsd <nic_swsd@...ltek.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH net-next 1/7] r8152: adjust rx_bottom

On Mon, Jan 19, 2015 at 6:48 PM, Hayes Wang <hayeswang@...ltek.com> wrote:
>  David Miller [mailto:davem@...emloft.net]
>> Sent: Tuesday, January 20, 2015 5:14 AM
> [...]
>> >> -               r8152_submit_rx(tp, agg, GFP_ATOMIC);
>> >> +               if (!ret) {
>> >> +                       ret = r8152_submit_rx(tp, agg, GFP_ATOMIC);
>> >> +               } else {
>> >> +                       urb->actual_length = 0;
>> >> +                       list_add_tail(&agg->list, next);
>> >
>> > Do you need a spin_lock_irqsave(&tp->rx_lock, flags) around this?
>>
>> Indeed, and rtl_start_rx() seems to also access agg->list without
>> proper locking.
>
> It is unnecessary because I deal with them in a local list_head. My steps are
>    1. Move the whole list from tp->rx_done to local rx_queue. (with spin lock)
>    2. dequeue/enqueue the lists in rx_queue.
>    3. Move the lists in rx_queue to tp->rx_done if it is necessary. (spin lock)
> For step 2, it wouldn't have race, because the list_head is local and no other
> function would change it. Therefore, I don't think it needs the spin lock.

Sorry guys, I think I made a mistake in my review and caused some
confusion/grief.

My mistake was getting the params to list_add_tail() backwards.  It's
list_add_tail(entry, head).  I saw list_add_tail(&agg->list, next) and
was fooled into thinking agg->list was the list getting appended with
the entry 'next'.  It's the opposite.  Duh.  So locking isn't needed
because the list is indeed local.

-scott
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