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

Marek Vasut
2026-04-07T21:23:51Z

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