Message-ID: <959c3cf5-c604-4b36-adb8-ee28ed4ef9c4@raritan.com>
Date: Tue, 30 Apr 2024 10:24:38 +0200
From: Ronald Wahl <ronald.wahl@...itan.com>
To: Marek Vasut <marex@...x.de>, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Subject: Re: [net,PATCH] net: ks8851: Queue RX packets in IRQ handler instead
 of disabling BHs

On 30.04.24 03:14, Marek Vasut wrote:
> Currently the driver uses local_bh_disable()/local_bh_enable() in its
> IRQ handler to avoid triggering net_rx_action() softirq on exit from
> netif_rx(). The net_rx_action() could trigger this driver .start_xmit
> callback, which is protected by the same lock as the IRQ handler, so
> calling the .start_xmit from netif_rx() from the IRQ handler critical
> section protected by the lock could lead to an attempt to claim the
> already claimed lock, and a hang.
>
> The local_bh_disable()/local_bh_enable() approach works only in case
> the IRQ handler is protected by a spinlock, but does not work if the
> IRQ handler is protected by mutex, i.e. this works for KS8851 with
> Parallel bus interface, but not for KS8851 with SPI bus interface.
>
> Remove the BH manipulation and instead of calling netif_rx() inside
> the IRQ handler code protected by the lock, queue all the received
> SKBs in the IRQ handler into a queue first, and once the IRQ handler
> exits the critical section protected by the lock, dequeue all the
> queued SKBs and push them all into netif_rx(). At this point, it is
> safe to trigger the net_rx_action() softirq, since the netif_rx()
> call is outside of the lock that protects the IRQ handler.
>
> Fixes: be0384bf599c ("net: ks8851: Handle softirqs at the end of IRQ thread to fix hang")
> Signed-off-by: Marek Vasut <marex@...x.de>
> ---
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: Ronald Wahl <ronald.wahl@...itan.com>
> Cc: Simon Horman <horms@...nel.org>
> Cc: netdev@...r.kernel.org

To me the code looks good. An iperf3 test shows that it now has even
slightly more throughput in my setup (two interconnected ks8851-spi).
Thanks for this fix!

Tested-by: Ronald Wahl <ronald.wahl@...itan.com>

> ---
> Note: This is basically what Jakub originally suggested in
>        https://patchwork.kernel.org/project/netdevbpf/patch/20240331142353.93792-2-marex@denx.de/#25785606
> ---
>   drivers/net/ethernet/micrel/ks8851.h        | 1 +
>   drivers/net/ethernet/micrel/ks8851_common.c | 8 ++++----
>   2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/ethernet/micrel/ks8851.h b/drivers/net/ethernet/micrel/ks8851.h
> index 31f75b4a67fd7..f311074ea13bc 100644
> --- a/drivers/net/ethernet/micrel/ks8851.h
> +++ b/drivers/net/ethernet/micrel/ks8851.h
> @@ -399,6 +399,7 @@ struct ks8851_net {
>
>       struct work_struct      rxctrl_work;
>
> +     struct sk_buff_head     rxq;
>       struct sk_buff_head     txq;
>       unsigned int            queued_len;
>
> diff --git a/drivers/net/ethernet/micrel/ks8851_common.c b/drivers/net/ethernet/micrel/ks8851_common.c
> index d4cdf3d4f5525..f7b48e596631f 100644
> --- a/drivers/net/ethernet/micrel/ks8851_common.c
> +++ b/drivers/net/ethernet/micrel/ks8851_common.c
> @@ -299,7 +299,7 @@ static void ks8851_rx_pkts(struct ks8851_net *ks)
>                                       ks8851_dbg_dumpkkt(ks, rxpkt);
>
>                               skb->protocol = eth_type_trans(skb, ks->netdev);
> -                             __netif_rx(skb);
> +                             skb_queue_tail(&ks->rxq, skb);
>
>                               ks->netdev->stats.rx_packets++;
>                               ks->netdev->stats.rx_bytes += rxlen;
> @@ -330,8 +330,6 @@ static irqreturn_t ks8851_irq(int irq, void *_ks)
>       unsigned long flags;
>       unsigned int status;
>
> -     local_bh_disable();
> -
>       ks8851_lock(ks, &flags);
>
>       status = ks8851_rdreg16(ks, KS_ISR);
> @@ -408,7 +406,8 @@ static irqreturn_t ks8851_irq(int irq, void *_ks)
>       if (status & IRQ_LCI)
>               mii_check_link(&ks->mii);
>
> -     local_bh_enable();
> +     while (!skb_queue_empty(&ks->rxq))
> +             netif_rx(skb_dequeue(&ks->rxq));
>
>       return IRQ_HANDLED;
>   }
> @@ -1189,6 +1188,7 @@ int ks8851_probe_common(struct net_device *netdev, struct device *dev,
>                                               NETIF_MSG_PROBE |
>                                               NETIF_MSG_LINK);
>
> +     skb_queue_head_init(&ks->rxq);
>       skb_queue_head_init(&ks->txq);
>
>       netdev->ethtool_ops = &ks8851_ethtool_ops;


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