[net,PATCH] net: ks8851: Reinstate disabling of BHs around IRQ handler

[Marek Vasut <marex@nabladev.com>]

If CONFIG_PREEMPT_RT=y is set AND the driver executes ks8851_irq() AND
KSZ_ISR register bit IRQ_RXI is set AND ks8851_rx_pkts() detects that
there are packets in the RX FIFO, then netdev_alloc_skb_ip_align() is
called to allocate SKBs. If netdev_alloc_skb_ip_align() is called with
BH enabled, local_bh_enable() at the end of netdev_alloc_skb_ip_align()
will call __local_bh_enable_ip(), which will call __do_softirq(), which
may trigger net_tx_action() softirq, which may ultimately call the xmit
callback ks8851_start_xmit_par(). The ks8851_start_xmit_par() will try
to lock struct ks8851_net_par .lock spinlock, which is already locked
by ks8851_irq() from which ks8851_start_xmit_par() was called. This
leads to a deadlock, which is reported by the kernel, including a trace
listed below.

Fix the problem by disabling BH around the IRQ handler, thus preventing
the net_tx_action() softirq from triggering during the IRQ handler. The
net_tx_action() softirq is now triggered at the end of the IRQ handler,
once all the other IRQ handler actions have been completed.

  __schedule from schedule_rtlock+0x1c/0x34
  schedule_rtlock from rtlock_slowlock_locked+0x538/0x894
  rtlock_slowlock_locked from rt_spin_lock+0x44/0x5c
  rt_spin_lock from ks8851_start_xmit_par+0x68/0x1a0
  ks8851_start_xmit_par from netdev_start_xmit+0x1c/0x40
  netdev_start_xmit from dev_hard_start_xmit+0xec/0x1b0
  dev_hard_start_xmit from sch_direct_xmit+0xb8/0x25c
  sch_direct_xmit from __qdisc_run+0x20c/0x4fc
  __qdisc_run from qdisc_run+0x1c/0x28
  qdisc_run from net_tx_action+0x1f4/0x244
  net_tx_action from handle_softirqs+0x1c0/0x29c
  handle_softirqs from __local_bh_enable_ip+0xdc/0xf4
  __local_bh_enable_ip from __netdev_alloc_skb+0x140/0x194
  __netdev_alloc_skb from ks8851_irq+0x348/0x4d8
  ks8851_irq from irq_thread_fn+0x24/0x64
  irq_thread_fn from irq_thread+0x110/0x1dc
  irq_thread from kthread+0x104/0x10c
  kthread from ret_from_fork+0x14/0x28

Fixes: e0863634bf9f ("net: ks8851: Queue RX packets in IRQ handler instead of disabling BHs")
Cc: stable@vger.kernel.org
Signed-off-by: Marek Vasut <marex@nabladev.com>
---
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Andrew Lunn <andrew+netdev@lunn.ch>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Ronald Wahl <ronald.wahl@raritan.com>
Cc: Yicong Hui <yiconghui@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
---
 drivers/net/ethernet/micrel/ks8851_common.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/micrel/ks8851_common.c b/drivers/net/ethernet/micrel/ks8851_common.c
index 8048770958d60..dadedea016fac 100644
--- a/drivers/net/ethernet/micrel/ks8851_common.c
+++ b/drivers/net/ethernet/micrel/ks8851_common.c
@@ -316,6 +316,7 @@ static irqreturn_t ks8851_irq(int irq, void *_ks)
 	unsigned int status;
 	struct sk_buff *skb;
 
+	local_bh_disable();
 	ks8851_lock(ks, &flags);
 
 	status = ks8851_rdreg16(ks, KS_ISR);
@@ -381,6 +382,7 @@ static irqreturn_t ks8851_irq(int irq, void *_ks)
 	if (status & IRQ_RXI)
 		while ((skb = __skb_dequeue(&rxq)))
 			netif_rx(skb);
+	local_bh_enable();
 
 	return IRQ_HANDLED;
 }
-- 
2.53.0