syzbot

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
kworker/0:1/9 is trying to acquire lock:
ffff8880186510b8 (&buf->lock){+.+.}-{3:3}, at: tty_buffer_flush+0x79/0x3f0 drivers/tty/tty_buffer.c:229

but task is already holding lock:
ffffffff8d126400 (console_lock){+.+.}-{0:0}, at: vc_SAK+0x28/0x220 drivers/tty/vt/vt_ioctl.c:985

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (console_lock){+.+.}-{0:0}:
       console_lock+0x164/0x1b0 kernel/printk/printk.c:2686
       con_flush_chars+0x4b/0x280 drivers/tty/vt/vt.c:3315
       __receive_buf drivers/tty/n_tty.c:1650 [inline]
       n_tty_receive_buf_common+0xc77/0x12d0 drivers/tty/n_tty.c:1745
       tiocsti+0x221/0x2a0 drivers/tty/tty_io.c:2291
       tty_ioctl+0x62e/0xdd0 drivers/tty/tty_io.c:2693
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:871 [inline]
       __se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
       do_syscall_x64 arch/x86/entry/common.c:46 [inline]
       do_syscall_64+0x55/0xa0 arch/x86/entry/common.c:76
       entry_SYSCALL_64_after_hwframe+0x68/0xd2

-> #1 (&tty->termios_rwsem){++++}-{3:3}:
       down_write+0x97/0x200 kernel/locking/rwsem.c:1573
       n_tty_flush_buffer+0x30/0x230 drivers/tty/n_tty.c:363
       tty_buffer_flush+0x328/0x3f0 drivers/tty/tty_buffer.c:241
       tty_ldisc_flush+0x6b/0xc0 drivers/tty/tty_ldisc.c:388
       tty_port_close_start+0x2da/0x540 drivers/tty/tty_port.c:660
       tty_port_close+0x2a/0x140 drivers/tty/tty_port.c:715
       tty_release+0x387/0x1600 drivers/tty/tty_io.c:1752
       __fput+0x234/0x970 fs/file_table.c:384
       __do_sys_close fs/open.c:1573 [inline]
       __se_sys_close+0x15f/0x220 fs/open.c:1558
       do_syscall_x64 arch/x86/entry/common.c:46 [inline]
       do_syscall_64+0x55/0xa0 arch/x86/entry/common.c:76
       entry_SYSCALL_64_after_hwframe+0x68/0xd2

-> #0 (&buf->lock){+.+.}-{3:3}:
       check_prev_add kernel/locking/lockdep.c:3134 [inline]
       check_prevs_add kernel/locking/lockdep.c:3253 [inline]
       validate_chain kernel/locking/lockdep.c:3869 [inline]
       __lock_acquire+0x2df1/0x7d40 kernel/locking/lockdep.c:5137
       lock_acquire+0x19e/0x420 kernel/locking/lockdep.c:5754
       __mutex_lock_common kernel/locking/mutex.c:603 [inline]
       __mutex_lock+0x136/0xcc0 kernel/locking/mutex.c:747
       tty_buffer_flush+0x79/0x3f0 drivers/tty/tty_buffer.c:229
       __do_SAK+0x135/0x6a0 drivers/tty/tty_io.c:3014
       vc_SAK+0x78/0x220 drivers/tty/vt/vt_ioctl.c:995
       process_one_work kernel/workqueue.c:2653 [inline]
       process_scheduled_works+0xa5d/0x15d0 kernel/workqueue.c:2730
       worker_thread+0xa55/0xfc0 kernel/workqueue.c:2811
       kthread+0x2fa/0x390 kernel/kthread.c:388
       ret_from_fork+0x48/0x80 arch/x86/kernel/process.c:152
       ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:293

other info that might help us debug this:

Chain exists of:
  &buf->lock --> &tty->termios_rwsem --> console_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(console_lock);
                               lock(&tty->termios_rwsem);
                               lock(console_lock);
  lock(&buf->lock);

 *** DEADLOCK ***

3 locks held by kworker/0:1/9:
 #0: ffff888017c70938 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:2628 [inline]
 #0: ffff888017c70938 ((wq_completion)events){+.+.}-{0:0}, at: process_scheduled_works+0x96f/0x15d0 kernel/workqueue.c:2730
 #1: ffffc900000e7d00 ((work_completion)(&vc_cons[currcons].SAK_work)){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:2628 [inline]
 #1: ffffc900000e7d00 ((work_completion)(&vc_cons[currcons].SAK_work)){+.+.}-{0:0}, at: process_scheduled_works+0x96f/0x15d0 kernel/workqueue.c:2730
 #2: ffffffff8d126400 (console_lock){+.+.}-{0:0}, at: vc_SAK+0x28/0x220 drivers/tty/vt/vt_ioctl.c:985

stack backtrace:
CPU: 0 PID: 9 Comm: kworker/0:1 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Workqueue: events vc_SAK
Call Trace:
 <TASK>
 dump_stack_lvl+0x18c/0x250 lib/dump_stack.c:106
 check_noncircular+0x2fc/0x400 kernel/locking/lockdep.c:2187
 check_prev_add kernel/locking/lockdep.c:3134 [inline]
 check_prevs_add kernel/locking/lockdep.c:3253 [inline]
 validate_chain kernel/locking/lockdep.c:3869 [inline]
 __lock_acquire+0x2df1/0x7d40 kernel/locking/lockdep.c:5137
 lock_acquire+0x19e/0x420 kernel/locking/lockdep.c:5754
 __mutex_lock_common kernel/locking/mutex.c:603 [inline]
 __mutex_lock+0x136/0xcc0 kernel/locking/mutex.c:747
 tty_buffer_flush+0x79/0x3f0 drivers/tty/tty_buffer.c:229
 __do_SAK+0x135/0x6a0 drivers/tty/tty_io.c:3014
 vc_SAK+0x78/0x220 drivers/tty/vt/vt_ioctl.c:995
 process_one_work kernel/workqueue.c:2653 [inline]
 process_scheduled_works+0xa5d/0x15d0 kernel/workqueue.c:2730
 worker_thread+0xa55/0xfc0 kernel/workqueue.c:2811
 kthread+0x2fa/0x390 kernel/kthread.c:388
 ret_from_fork+0x48/0x80 arch/x86/kernel/process.c:152
 ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:293
 </TASK>