Subject: Re: [PATCH wpan-next v2 13/14] net: mac802154: Introduce a tx queue flushing mechanism

From: Miquel Raynal <[email-masked]>
Date: Thu, 3 Mar 2022 19:17:23 +0100

Hi Alexander,

[email-masked] wrote on Sun, 20 Feb 2022 18:49:06 -0500:

> Hi,
> 
> On Mon, Feb 7, 2022 at 9:48 AM Miquel Raynal <[email-masked]> wrote:
> >
> > Right now we are able to stop a queue but we have no indication if a
> > transmission is ongoing or not.
> >
> > Thanks to recent additions, we can track the number of ongoing
> > transmissions so we know if the last transmission is over. Adding on top
> > of it an internal wait queue also allows to be woken up asynchronously
> > when this happens. If, beforehands, we marked the queue to be held and
> > stopped it, we end up flushing and stopping the tx queue.
> >
> > Thanks to this feature, we will soon be able to introduce a synchronous
> > transmit API.
> >
> > Signed-off-by: Miquel Raynal <[email-masked]>
> > ---
> >  include/net/cfg802154.h      |  1 +
> >  net/ieee802154/core.c        |  1 +
> >  net/mac802154/cfg.c          |  5 ++---
> >  net/mac802154/ieee802154_i.h |  1 +
> >  net/mac802154/tx.c           | 11 ++++++++++-
> >  net/mac802154/util.c         |  3 ++-
> >  6 files changed, 17 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/net/cfg802154.h b/include/net/cfg802154.h
> > index 043d8e4359e7..0d385a214da3 100644
> > --- a/include/net/cfg802154.h
> > +++ b/include/net/cfg802154.h
> > @@ -217,6 +217,7 @@ struct wpan_phy {
> >         /* Transmission monitoring and control */
> >         atomic_t ongoing_txs;
> >         atomic_t hold_txs;
> > +       wait_queue_head_t sync_txq;
> >
> >         char priv[] __aligned(NETDEV_ALIGN);
> >  };
> > diff --git a/net/ieee802154/core.c b/net/ieee802154/core.c
> > index de259b5170ab..0953cacafbff 100644
> > --- a/net/ieee802154/core.c
> > +++ b/net/ieee802154/core.c
> > @@ -129,6 +129,7 @@ wpan_phy_new(const struct cfg802154_ops *ops, size_t priv_size)
> >         wpan_phy_net_set(&rdev->wpan_phy, &init_net);
> >
> >         init_waitqueue_head(&rdev->dev_wait);
> > +       init_waitqueue_head(&rdev->wpan_phy.sync_txq);
> >
> >         return &rdev->wpan_phy;
> >  }
> > diff --git a/net/mac802154/cfg.c b/net/mac802154/cfg.c
> > index e8aabf215286..da94aaa32fcb 100644
> > --- a/net/mac802154/cfg.c
> > +++ b/net/mac802154/cfg.c
> > @@ -46,8 +46,7 @@ static int ieee802154_suspend(struct wpan_phy *wpan_phy)
> >         if (!local->open_count)
> >                 goto suspend;
> >
> > -       atomic_inc(&wpan_phy->hold_txs);
> > -       ieee802154_stop_queue(&local->hw);
> > +       ieee802154_sync_and_stop_tx(local);
> >         synchronize_net();
> >
> >         /* stop hardware - this must stop RX */
> > @@ -73,7 +72,7 @@ static int ieee802154_resume(struct wpan_phy *wpan_phy)
> >                 return ret;
> >
> >  wake_up:
> > -       if (!atomic_dec_and_test(&wpan_phy->hold_txs))
> > +       if (!atomic_read(&wpan_phy->hold_txs))
> >                 ieee802154_wake_queue(&local->hw);
> >         local->suspended = false;
> >         return 0;
> > diff --git a/net/mac802154/ieee802154_i.h b/net/mac802154/ieee802154_i.h
> > index 56fcd7ef5b6f..295c9ce091e1 100644
> > --- a/net/mac802154/ieee802154_i.h
> > +++ b/net/mac802154/ieee802154_i.h
> > @@ -122,6 +122,7 @@ extern struct ieee802154_mlme_ops mac802154_mlme_wpan;
> >
> >  void ieee802154_rx(struct ieee802154_local *local, struct sk_buff *skb);
> >  void ieee802154_xmit_sync_worker(struct work_struct *work);
> > +void ieee802154_sync_and_stop_tx(struct ieee802154_local *local);
> >  netdev_tx_t
> >  ieee802154_monitor_start_xmit(struct sk_buff *skb, struct net_device *dev);
> >  netdev_tx_t
> > diff --git a/net/mac802154/tx.c b/net/mac802154/tx.c
> > index abd9a057521e..06ae2e6cea43 100644
> > --- a/net/mac802154/tx.c
> > +++ b/net/mac802154/tx.c
> > @@ -47,7 +47,8 @@ void ieee802154_xmit_sync_worker(struct work_struct *work)
> >                 ieee802154_wake_queue(&local->hw);
> >
> >         kfree_skb(skb);
> > -       atomic_dec(&local->phy->ongoing_txs);
> > +       if (!atomic_dec_and_test(&local->phy->ongoing_txs))
> > +               wake_up(&local->phy->sync_txq);
> >         netdev_dbg(dev, "transmission failed\n");
> >  }
> >
> > @@ -117,6 +118,14 @@ ieee802154_hot_tx(struct ieee802154_local *local, struct sk_buff *skb)
> >         return ieee802154_tx(local, skb);
> >  }
> >
> > +void ieee802154_sync_and_stop_tx(struct ieee802154_local *local)
> > +{
> > +       atomic_inc(&local->phy->hold_txs);
> > +       ieee802154_stop_queue(&local->hw);
> > +       wait_event(local->phy->sync_txq, !atomic_read(&local->phy->ongoing_txs));
> > +       atomic_dec(&local->phy->hold_txs);  
> 
> In my opinion this _still_ races as I mentioned earlier. You need to
> be sure that if you do ieee802154_stop_queue() that no ieee802154_tx()
> or hot_tx() is running at this time. Look into the function I
> mentioned earlier "?netif_tx_disable()?".

I think now I get the problem, but I am having troubles understanding
the logic in netif_tx_disable(), or should I say, the idea that I
should adapt to our situation.

I understand that we should make sure the following situation does not
happen:
- ieee802154_subif_start_xmit() is called
- ieee802154_subif_start_xmit() is called again
- ieee802154_tx() get's executed once and stops the queue
- ongoing_txs gets incremented once
- the first transfer finishes and ongoing_txs gets decremented
- the tx queue is supposedly empty by the current series while
  the second transfer requested earlier has not yet been processed and
  will definitely be tried in a short moment.

I don't find a pretty solution for that. Is your suggestion to use the
netdev tx_global_lock? If yes, then, how? Because it does not appear
clear to me how we should tackle this issue.

In the mean time, I believe the first half of the series is now big
enough to be sent aside given the number of additional commits that
have popped up following your last review :)

Thanks,
Miquèl