[PATCH net v2 2/2] net: ks8851: Handle softirqs at the end of IRQ thread to fix hang

[Marek Vasut <marex@denx.de>]

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: Ratheesh Kannoth <rkannoth@marvell.com>
Cc: Ronald Wahl <ronald.wahl@raritan.com>
Cc: Simon Horman <horms@kernel.org>
Cc: netdev@vger.kernel.org
---
V2: - Drop the need_bh_off test, the test was always true
    - Fill in 'net' subject prefix
---
 drivers/net/ethernet/micrel/ks8851_common.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/micrel/ks8851_common.c b/drivers/net/ethernet/micrel/ks8851_common.c
index 896d43bb8883d..d4cdf3d4f5525 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;
@@ -330,6 +330,8 @@ 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);
@@ -406,6 +408,8 @@ static irqreturn_t ks8851_irq(int irq, void *_ks)
 	if (status & IRQ_LCI)
 		mii_check_link(&ks->mii);
 
+	local_bh_enable();
+
 	return IRQ_HANDLED;
 }
 
-- 
2.43.0