[v5.15] possible deadlock in digi_set_modem_signals

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: aabd761612db Linux 5.15.215
git tree: linux-5.15.y
console output: https://syzkaller.appspot.com/x/log.txt?x=16a03a79580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f161cbc9aef65db0
dashboard link: https://syzkaller.appspot.com/bug?extid=b15725e156b064496594
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/f636f56d3662/disk-aabd7616.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f1a46620f40c/vmlinux-aabd7616.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9b78bb48b022/bzImage-aabd7616.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b15725...@syzkaller.appspotmail.com

digi_2 ttyUSB0: digi_write_oob_command: usb_submit_urb failed, ret=-19
============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.2.7/4286 is trying to acquire lock:
ffff888078766818 (&priv->dp_port_lock){..-.}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
ffff888078766818 (&priv->dp_port_lock){..-.}-{2:2}, at: digi_set_modem_signals+0x1a1/0x860 drivers/usb/serial/digi_acceleport.c:528

but task is already holding lock:
ffff88802bd00418 (&priv->dp_port_lock){..-.}-{2:2}, at: digi_set_modem_signals+0x194/0x860 drivers/usb/serial/digi_acceleport.c:527

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&priv->dp_port_lock);
lock(&priv->dp_port_lock);

*** DEADLOCK ***

May be due to missing lock nesting notation

2 locks held by syz.2.7/4286:
#0: ffff8880212351c0 (&tty->legacy_mutex){+.+.}-{3:3}, at: tty_init_dev+0x6f/0x470 drivers/tty/tty_io.c:1430
#1: ffff88802bd00418 (&priv->dp_port_lock){..-.}-{2:2}, at: digi_set_modem_signals+0x194/0x860 drivers/usb/serial/digi_acceleport.c:527

stack backtrace:
CPU: 0 PID: 4286 Comm: syz.2.7 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
<TASK>
dump_stack_lvl+0x188/0x250 lib/dump_stack.c:106
__lock_acquire+0x1247/0x7b50 kernel/locking/lockdep.c:-1
lock_acquire+0x19e/0x400 kernel/locking/lockdep.c:5623
__raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
_raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:154
spin_lock include/linux/spinlock.h:364 [inline]
digi_set_modem_signals+0x1a1/0x860 drivers/usb/serial/digi_acceleport.c:528
tty_port_raise_dtr_rts drivers/tty/tty_port.c:420 [inline]
tty_port_block_til_ready+0x375/0x8b0 drivers/tty/tty_port.c:504
tty_open+0x485/0xcd0 drivers/tty/tty_io.c:2165
chrdev_open+0x585/0x5f0 fs/char_dev.c:414
do_dentry_open+0x7f7/0xf90 fs/open.c:826
do_open fs/namei.c:3616 [inline]
path_openat+0x2718/0x2fb0 fs/namei.c:3750
do_filp_open+0x1df/0x400 fs/namei.c:3777
do_sys_openat2+0x14b/0x500 fs/open.c:1255
do_sys_open fs/open.c:1271 [inline]
__do_sys_openat fs/open.c:1287 [inline]
__se_sys_openat fs/open.c:1282 [inline]
__x64_sys_openat+0x135/0x160 fs/open.c:1282
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7fa7345510d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fa7327a9028 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007fa7347d8fa0 RCX: 00007fa7345510d9
RDX: 0000000000000002 RSI: 0000200000000700 RDI: ffffffffffffff9c
RBP: 00007fa7345e8024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa7347d9038 R14: 00007fa7347d8fa0 R15: 00007ffce18e2f78
</TASK>
digi_2 ttyUSB0: digi_set_modem_signals: usb_submit_urb failed, ret=-19


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup