netdev - Re: [PATCH net 1/4] bnxt: don't lock the tx queue from napi poll

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Date:   Wed, 11 Aug 2021 13:13:22 -0700
From:   Edwin Peer <edwin.peer@...adcom.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Michael Chan <michael.chan@...adcom.com>,
        Jeffrey Huang <huangjw@...adcom.com>,
        Eddie Wai <eddie.wai@...adcom.com>,
        Prashant Sreedharan <prashant@...adcom.com>,
        Andrew Gospodarek <gospo@...adcom.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net 1/4] bnxt: don't lock the tx queue from napi poll

On Wed, Aug 11, 2021 at 12:32 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> We can't take the tx lock from the napi poll routine, because
> netpoll can poll napi at any moment, including with the tx lock
> already held.
>
> It seems that the tx lock is only protecting against the disable
> path, appropriate barriers are already in place to make sure
> cleanup can safely run concurrently with start_xmit. I don't see
> any other reason why 'stopped && avail > thresh' needs to be
> re-checked under the lock.
>
> Remove the tx lock and use synchronize_net() to make sure
> closing the device does not race we restarting the queues.
> Annotate accesses to dev_state against data races.
>
> Fixes: c0c050c58d84 ("bnxt_en: New Broadcom ethernet driver.")
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
>  drivers/net/ethernet/broadcom/bnxt/bnxt.c | 21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt.c b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
> index 865fcb8cf29f..07827d6b0fec 100644
> --- a/drivers/net/ethernet/broadcom/bnxt/bnxt.c
> +++ b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
> @@ -730,15 +730,10 @@ static void bnxt_tx_int(struct bnxt *bp, struct bnxt_napi *bnapi, int nr_pkts)
>          */
>         smp_mb();
>
> -       if (unlikely(netif_tx_queue_stopped(txq)) &&
> -           (bnxt_tx_avail(bp, txr) > bp->tx_wake_thresh)) {
> -               __netif_tx_lock(txq, smp_processor_id());
> -               if (netif_tx_queue_stopped(txq) &&
> -                   bnxt_tx_avail(bp, txr) > bp->tx_wake_thresh &&
> -                   txr->dev_state != BNXT_DEV_STATE_CLOSING)
> -                       netif_tx_wake_queue(txq);
> -               __netif_tx_unlock(txq);
> -       }
> +       if (netif_tx_queue_stopped(txq) &&

Probably worth retaining the unlikely() that the original outer check had.

> +           bnxt_tx_avail(bp, txr) > bp->tx_wake_thresh &&
> +           READ_ONCE(txr->dev_state) != BNXT_DEV_STATE_CLOSING)
> +               netif_tx_wake_queue(txq);
>  }
>
>  static struct page *__bnxt_alloc_rx_page(struct bnxt *bp, dma_addr_t *mapping,
> @@ -9264,9 +9259,11 @@ void bnxt_tx_disable(struct bnxt *bp)
>         if (bp->tx_ring) {
>                 for (i = 0; i < bp->tx_nr_rings; i++) {
>                         txr = &bp->tx_ring[i];
> -                       txr->dev_state = BNXT_DEV_STATE_CLOSING;
> +                       WRITE_ONCE(txr->dev_state, BNXT_DEV_STATE_CLOSING);
>                 }
>         }
> +       /* Make sure napi polls see @dev_state change */
> +       synchronize_net();
>         /* Drop carrier first to prevent TX timeout */
>         netif_carrier_off(bp->dev);
>         /* Stop all TX queues */
> @@ -9280,8 +9277,10 @@ void bnxt_tx_enable(struct bnxt *bp)
>
>         for (i = 0; i < bp->tx_nr_rings; i++) {
>                 txr = &bp->tx_ring[i];
> -               txr->dev_state = 0;
> +               WRITE_ONCE(txr->dev_state, 0);
>         }
> +       /* Make sure napi polls see @dev_state change */
> +       synchronize_net();
>         netif_tx_wake_all_queues(bp->dev);
>         if (bp->link_info.link_up)
>                 netif_carrier_on(bp->dev);
> --
> 2.31.1

Regards,
Edwin Peer