A potential deadlock due to A-B/B-A deadlock exists between the NFC core and the RFKill subsystem, involving the NFC device lock and the rfkill_global_mutex. This issue is particularly visible on PREEMPT_RT kernels, which can report the following warning: | rtmutex deadlock detected | WARNING: CPU: 0 PID: 22729 at kernel/locking/rtmutex.c:1674 rt_mutex_handle_deadlock+0x68/0xec kernel/locking/rtmutex.c:-1 | Modules linked in: | CPU: 0 UID: 0 PID: 22729 Comm: syz.7.2187 Kdump: loaded Not tainted 6.17.0-rc1-00001-g1149a5db27c8-dirty #55 PREEMPT_RT | Hardware name: QEMU KVM Virtual Machine, BIOS 2025.02-8ubuntu1 06/11/2025 | pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--) | pc : rt_mutex_handle_deadlock+0x68/0xec kernel/locking/rtmutex.c:-1 | lr : rt_mutex_handle_deadlock+0x40/0xec kernel/locking/rtmutex.c:1674 | sp : ffff8000967c7720 | x29: ffff8000967c7720 x28: 1fffe0001946d182 x27: dfff800000000000 | x26: 0000000000000001 x25: 0000000000000003 x24: 1fffe0001946d00b | x23: 1fffe0001946d182 x22: ffff80008aec8940 x21: dfff800000000000 | x20: ffff0000ca368058 x19: ffff0000ca368c10 x18: ffff80008af6b6e0 | x17: 1fffe000590b8088 x16: ffff80008046cc08 x15: 0000000000000001 | x14: 1fffe000590ba990 x13: 0000000000000000 x12: 0000000000000000 | x11: ffff6000590ba991 x10: 0000000000000002 x9 : 0fe446e029bcfe00 | x8 : 0000000000000000 x7 : 0000000000000000 x6 : 000000000000003f | x5 : 0000000000000001 x4 : 0000000000001000 x3 : ffff800080503efc | x2 : 0000000000000001 x1 : 0000000000000001 x0 : 0000000000000001 | Call trace: | rt_mutex_handle_deadlock+0x68/0xec kernel/locking/rtmutex.c:-1 (P) | __rt_mutex_slowlock+0x1cc/0x480 kernel/locking/rtmutex.c:1734 | __rt_mutex_slowlock_locked kernel/locking/rtmutex.c:1760 [inline] | rt_mutex_slowlock+0x140/0x21c kernel/locking/rtmutex.c:1800 | __rt_mutex_lock kernel/locking/rtmutex.c:1815 [inline] | __mutex_lock_common kernel/locking/rtmutex_api.c:536 [inline] | mutex_lock+0xf0/0x10c kernel/locking/rtmutex_api.c:603 | device_lock include/linux/device.h:911 [inline] | nfc_dev_down net/nfc/core.c:143 [inline] | nfc_rfkill_set_block+0x48/0x2a4 net/nfc/core.c:179 | rfkill_set_block+0x184/0x364 net/rfkill/core.c:346 | rfkill_fop_write+0x4dc/0x624 net/rfkill/core.c:1301 | vfs_write+0x2b8/0xa30 fs/read_write.c:684 | ksys_write+0x120/0x210 fs/read_write.c:738 | __do_sys_write fs/read_write.c:749 [inline] | __se_sys_write fs/read_write.c:746 [inline] | __arm64_sys_write+0x7c/0x90 fs/read_write.c:746 | __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline] | invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49 | el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132 | do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151 | el0_svc+0x40/0x140 arch/arm64/kernel/entry-common.c:879 | el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:898 | el0t_64_sync+0x1ac/0x1b0 arch/arm64/kernel/entry.S:596 The scenario is as follows: Task A (rfkill_fop_write): 1. Acquires rfkill_global_mutex. 2. Iterates devices and calls rfkill_set_block() -> nfc_rfkill_set_block() -> nfc_dev_down(). 3. Tries to acquire NFC device_lock. Task B (nfc_unregister_device): 1. Acquires NFC device_lock. 2. Calls rfkill_unregister(). 3. Tries to acquire rfkill_global_mutex. Task A waits for the device_lock held by Task B, while Task B waits for the rfkill_global_mutex held by Task A. To fix this, move the calls to rfkill_unregister() and rfkill_destroy() outside the device_lock critical section in nfc_unregister_device(). We ensure this is safe by first acquiring the device_lock, setting the shutting_down flag (which prevents races with nfc_dev_down()), stashing the rfkill pointer in a local variable, nullifying the pointer in the nfc_dev structure, and then releasing the device_lock before calling the rfkill unregister functions. This breaks the lock inversion. Signed-off-by: Yunseong Kim --- net/nfc/core.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/net/nfc/core.c b/net/nfc/core.c index ae1c842f9c64..c8dc6414514b 100644 --- a/net/nfc/core.c +++ b/net/nfc/core.c @@ -1154,6 +1154,7 @@ EXPORT_SYMBOL(nfc_register_device); void nfc_unregister_device(struct nfc_dev *dev) { int rc; + struct rfkill *rfk = NULL; pr_debug("dev_name=%s\n", dev_name(&dev->dev)); @@ -1163,14 +1164,18 @@ void nfc_unregister_device(struct nfc_dev *dev) "was removed\n", dev_name(&dev->dev)); device_lock(&dev->dev); + dev->shutting_down = true; if (dev->rfkill) { - rfkill_unregister(dev->rfkill); - rfkill_destroy(dev->rfkill); + rfk = dev->rfkill; dev->rfkill = NULL; } - dev->shutting_down = true; device_unlock(&dev->dev); + if (rfk) { + rfkill_unregister(rfk); + rfkill_destroy(rfk); + } + if (dev->ops->check_presence) { timer_delete_sync(&dev->check_pres_timer); cancel_work_sync(&dev->check_pres_work); -- 2.50.0