[PATCH 6.12 315/567] smc91x: fix broken irq-context in PREEMPT_RT

Greg Kroah-Hartman
2026-01-06T17:32:37Z

From: Yeoreum Yun <yeoreum.yun@arm.com>

[ Upstream commit 6402078bd9d1ed46e79465e1faaa42e3458f8a33 ]

When smc91x.c is built with PREEMPT_RT, the following splat occurs
in FVP_RevC:

[ 13.055000] smc91x LNRO0003:00 eth0: link up, 10Mbps, half-duplex, lpa 0x0000
[ 13.062137] BUG: workqueue leaked atomic, lock or RCU: kworker/2:1[106]
[ 13.062137] preempt=0x00000000 lock=0->0 RCU=0->1 workfn=mld_ifc_work
[ 13.062266] C
** replaying previous printk message **
[ 13.062266] CPU: 2 UID: 0 PID: 106 Comm: kworker/2:1 Not tainted 6.18.0-dirty #179 PREEMPT_{RT,(full)}
[ 13.062353] Hardware name: , BIOS
[ 13.062382] Workqueue: mld mld_ifc_work
[ 13.062469] Call trace:
[ 13.062494] show_stack+0x24/0x40 (C)
[ 13.062602] __dump_stack+0x28/0x48
[ 13.062710] dump_stack_lvl+0x7c/0xb0
[ 13.062818] dump_stack+0x18/0x34
[ 13.062926] process_scheduled_works+0x294/0x450
[ 13.063043] worker_thread+0x260/0x3d8
[ 13.063124] kthread+0x1c4/0x228
[ 13.063235] ret_from_fork+0x10/0x20

This happens because smc_special_trylock() disables IRQs even on PREEMPT_RT,
but smc_special_unlock() does not restore IRQs on PREEMPT_RT.
The reason is that smc_special_unlock() calls spin_unlock_irqrestore(),
and rcu_read_unlock_bh() in __dev_queue_xmit() cannot invoke
rcu_read_unlock() through __local_bh_enable_ip() when current->softirq_disable_cnt becomes zero.

To address this issue, replace smc_special_trylock() with spin_trylock_irqsave().

Fixes: 342a93247e08 ("locking/spinlock: Provide RT variant header: <linux/spinlock_rt.h>")
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251217085115.1730036-1-yeoreum.yun@arm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/net/ethernet/smsc/smc91x.c | 10 +---------
1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/smsc/smc91x.c b/drivers/net/ethernet/smsc/smc91x.c
index a5e23e2da90f..953a1d22e60a 100644
--- a/drivers/net/ethernet/smsc/smc91x.c
+++ b/drivers/net/ethernet/smsc/smc91x.c
@@ -516,15 +516,7 @@ static inline void smc_rcv(struct net_device *dev)
* any other concurrent access and C would always interrupt B. But life
* isn't that easy in a SMP world...
*/
-#define smc_special_trylock(lock, flags) \
-({ \
- int __ret; \
- local_irq_save(flags); \
- __ret = spin_trylock(lock); \
- if (!__ret) \
- local_irq_restore(flags); \
- __ret; \
-})
+#define smc_special_trylock(lock, flags) spin_trylock_irqsave(lock, flags)
#define smc_special_lock(lock, flags) spin_lock_irqsave(lock, flags)
#define smc_special_unlock(lock, flags) spin_unlock_irqrestore(lock, flags)
#else
--
2.51.0