netdev - Re: [PATCH] net: mac802154: Fix racy device stats updates by DEV_STATS_INC() and DEV_STATS

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Date: Wed, 5 Jun 2024 17:54:59 -0400
From: Alexander Aring <aahringo@...hat.com>
To: Stefan Schmidt <stefan@...enfreihafen.org>
Cc: Jakub Kicinski <kuba@...nel.org>, Yunshui Jiang <jiangyunshui@...inos.cn>, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	linux-wpan@...r.kernel.org, alex.aring@...il.com, miquel.raynal@...tlin.com, 
	davem@...emloft.net
Subject: Re: [PATCH] net: mac802154: Fix racy device stats updates by
 DEV_STATS_INC() and DEV_STATS_ADD()

Hi,

On Wed, Jun 5, 2024 at 4:02 PM Stefan Schmidt <stefan@...enfreihafen.org> wrote:
>
> Hello Jakub, Alex,
>
> On 04.06.24 15:52, Alexander Aring wrote:
> > Hi,
> >
> > On Mon, Jun 3, 2024 at 7:56 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >>
> >> On Mon, 3 Jun 2024 11:33:28 +0200 Stefan Schmidt wrote:
> >>> Hello.
> >>>
> >>> On 31.05.24 10:07, Yunshui Jiang wrote:
> >>>> mac802154 devices update their dev->stats fields locklessly. Therefore
> >>>> these counters should be updated atomically. Adopt SMP safe DEV_STATS_INC()
> >>>> and DEV_STATS_ADD() to achieve this.
> >>>>
> >>>> Signed-off-by: Yunshui Jiang <jiangyunshui@...inos.cn>
> >>>> ---
> >>>>    net/mac802154/tx.c | 8 ++++----
> >>>>    1 file changed, 4 insertions(+), 4 deletions(-)
> >>>>
> >>>> diff --git a/net/mac802154/tx.c b/net/mac802154/tx.c
> >>>> index 2a6f1ed763c9..6fbed5bb5c3e 100644
> >>>> --- a/net/mac802154/tx.c
> >>>> +++ b/net/mac802154/tx.c
> >>>> @@ -34,8 +34,8 @@ void ieee802154_xmit_sync_worker(struct work_struct *work)
> >>>>      if (res)
> >>>>              goto err_tx;
> >>>>
> >>>> -   dev->stats.tx_packets++;
> >>>> -   dev->stats.tx_bytes += skb->len;
> >>>> +   DEV_STATS_INC(dev, tx_packets);
> >>>> +   DEV_STATS_ADD(dev, tx_bytes, skb->len);
> >>>>
> >>>>      ieee802154_xmit_complete(&local->hw, skb, false);
> >>>>
> >>>> @@ -90,8 +90,8 @@ ieee802154_tx(struct ieee802154_local *local, struct sk_buff *skb)
> >>>>              if (ret)
> >>>>                      goto err_wake_netif_queue;
> >>>>
> >>>> -           dev->stats.tx_packets++;
> >>>> -           dev->stats.tx_bytes += len;
> >>>> +           DEV_STATS_INC(dev, tx_packets);
> >>>> +           DEV_STATS_ADD(dev, tx_bytes, len);
> >>>>      } else {
> >>>>              local->tx_skb = skb;
> >>>>              queue_work(local->workqueue, &local->sync_tx_work);
> >>>
> >>> This patch has been applied to the wpan tree and will be
> >>> part of the next pull request to net. Thanks!
> >>
> >> Hi! I haven't looked in detail, but FWIW
> >>
> >> $ git grep LLTX net/mac802154/
> >> $
> >>
> >> and similar patch from this author has been rejected:
> >>
> >> https://lore.kernel.org/all/CANn89iLPYoOjMxNjBVHY7GwPFBGuxwRoM9gZZ-fWUUYFYjM1Uw@mail.gmail.com/
> >
> > In the case of ieee802154_tx() yes the tx lock is held so it's like
> > what the mentioned link says. The workqueue is an ordered workqueue
> > and you either have the driver async xmit option (the preferred
> > option) or the driver sync xmit callback on a per driver (implies per
> > interface) basis.
>
> When I reviewed and applied this I did indeed not realize the ordered
> workqueue making this unnecessary.
>
> > I also don't see why there is currently a problem with the current
> > non-atomic way.
>
> For me this looks more like a wrapper that could avoid future problems
> for no cost. I would not mind if the patch stays. But you are right, its
> not strictly needed. Want me to back it out again?

it's fine for me. I think atomic ops, depending on $ARCH will mess up
your pipelines in some kind of way but it is better than using locks
of course (but locks are here required for other things).

- Alex