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

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Message-ID: <CAB_54W5ao0b6QE7E_uXFeorbn6UjB6NV4emtibqswL4iXYEfng@mail.gmail.com>
Date:   Sun, 20 Feb 2022 18:49:06 -0500
From:   Alexander Aring <alex.aring@...il.com>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Stefan Schmidt <stefan@...enfreihafen.org>,
        linux-wpan - ML <linux-wpan@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
        David Girault <david.girault@...vo.com>,
        Romuald Despres <romuald.despres@...vo.com>,
        Frederic Blain <frederic.blain@...vo.com>,
        Nicolas Schodet <nico@...fr.eu.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH wpan-next v2 13/14] net: mac802154: Introduce a tx queue
 flushing mechanism

Hi,

On Mon, Feb 7, 2022 at 9:48 AM Miquel Raynal <miquel.raynal@...tlin.com> 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 <miquel.raynal@...tlin.com>
> ---
>  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()?".

- Alex