Re: [PATCH 2/2] net: ks8851: Handle softirqs at the end of IRQ thread to fix hang

[Ratheesh Kannoth <rkannoth@marvell.com>]

On 2024-03-31 at 19:51:46, Marek Vasut (marex@denx.de) wrote:
> The ks8851_irq() thread may call ks8851_rx_pkts() in case there are
> any packets in the MAC FIFO, which calls netif_rx(). This netif_rx()
> implementation is guarded by local_bh_disable() and local_bh_enable().
> The local_bh_enable() may call do_softirq() to run softirqs in case
> any are pending. One of the softirqs is net_rx_action, which ultimately
> reaches the driver .start_xmit callback. If that happens, the system
> hangs. The entire call chain is below:
>
> ks8851_start_xmit_par from netdev_start_xmit
> netdev_start_xmit from dev_hard_start_xmit
> dev_hard_start_xmit from sch_direct_xmit
> sch_direct_xmit from __dev_queue_xmit
> __dev_queue_xmit from __neigh_update
> __neigh_update from neigh_update
> neigh_update from arp_process.constprop.0
> arp_process.constprop.0 from __netif_receive_skb_one_core
> __netif_receive_skb_one_core from process_backlog
> process_backlog from __napi_poll.constprop.0
> __napi_poll.constprop.0 from net_rx_action
> net_rx_action from __do_softirq
> __do_softirq from call_with_stack
> call_with_stack from do_softirq
> do_softirq from __local_bh_enable_ip
> __local_bh_enable_ip from netif_rx
> netif_rx from ks8851_irq
> ks8851_irq from irq_thread_fn
> irq_thread_fn from irq_thread
> irq_thread from kthread
> kthread from ret_from_fork
>
> The hang happens because ks8851_irq() first locks a spinlock in
> ks8851_par.c ks8851_lock_par() spin_lock_irqsave(&ksp->lock, ...)
> and with that spinlock locked, calls netif_rx(). Once the execution
> reaches ks8851_start_xmit_par(), it calls ks8851_lock_par() again
> which attempts to claim the already locked spinlock again, and the
> hang happens.
>
> Move the do_softirq() call outside of the spinlock protected section
> of ks8851_irq() by disabling BHs around the entire spinlock protected
> section of ks8851_irq() handler. Place local_bh_enable() outside of
> the spinlock protected section, so that it can trigger do_softirq()
> without the ks8851_par.c ks8851_lock_par() spinlock being held, and
> safely call ks8851_start_xmit_par() without attempting to lock the
> already locked spinlock.
>
> Since ks8851_irq() is protected by local_bh_disable()/local_bh_enable()
> now, replace netif_rx() with __netif_rx() which is not duplicating the
> local_bh_disable()/local_bh_enable() calls.
>
> Fixes: 797047f875b5 ("net: ks8851: Implement Parallel bus operations")
> Signed-off-by: Marek Vasut <marex@denx.de>
> ---
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Ronald Wahl <ronald.wahl@raritan.com>
> Cc: Simon Horman <horms@kernel.org>
> Cc: netdev@vger.kernel.org
> ---
>  drivers/net/ethernet/micrel/ks8851_common.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/micrel/ks8851_common.c b/drivers/net/ethernet/micrel/ks8851_common.c
> index 896d43bb8883d..b6b727e651f3d 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);
> +				__netif_rx(skb);
>
>  				ks->netdev->stats.rx_packets++;
>  				ks->netdev->stats.rx_bytes += rxlen;
> @@ -325,11 +325,15 @@ static void ks8851_rx_pkts(struct ks8851_net *ks)
>   */
>  static irqreturn_t ks8851_irq(int irq, void *_ks)
>  {
> +	bool need_bh_off = !(hardirq_count() | softirq_count());
IMO, in_task() macro would be better.

>  	struct ks8851_net *ks = _ks;
>  	unsigned handled = 0;
>  	unsigned long flags;
>  	unsigned int status;
>
> +	if (need_bh_off)
> +		local_bh_disable();
This threaded irq's thread function (ks8851_irq()) will always run in process context, right ?
Do you need "if(need_bh_off)" loop?

> +
>  	ks8851_lock(ks, &flags);
>
>  	status = ks8851_rdreg16(ks, KS_ISR);
> @@ -406,6 +410,9 @@ static irqreturn_t ks8851_irq(int irq, void *_ks)
>  	if (status & IRQ_LCI)
>  		mii_check_link(&ks->mii);
>
> +	if (need_bh_off)
> +		local_bh_enable();
> +
>  	return IRQ_HANDLED;
>  }
>
> --
> 2.43.0
>