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

[Stefan Schmidt]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > >    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@xxxxxxxxxxxxxx/
>
> 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?

regards
Stefan Schmidt