On 2018-07-09 11:55:08 [+0200], Esben Haabendal wrote:
> I am using the following patch.
> Not sure if it is worth proposing it for mainline inclusion, though.
> RFC:
| ======================================================
| WARNING: possible circular locking dependency detected
| 4.16.18-rt9+ #187 Not tainted
| ------------------------------------------------------
| sshd/3205 is trying to acquire lock:
| (&buf->lock){+.+.}, at: [< (ptrval)>] flush_to_ldisc+0x1e/0xa0
|
| but task is already holding lock:
| (&ldata->output_lock){+.+.}, at: [< (ptrval)>] n_tty_write+0x12a/0x480
|
| which lock already depends on the new lock.
|
|
| the existing dependency chain (in reverse order) is:
|
| -> #2 (&ldata->output_lock){+.+.}:
| _mutex_lock+0x26/0x40
| n_tty_write+0x12a/0x480
| tty_write+0x1b3/0x320
| redirected_tty_write+0x9a/0xb0
| do_iter_write+0x159/0x1a0
| vfs_writev+0x93/0x110
| do_writev+0x5f/0xf0
| SyS_writev+0xb/0x10
| do_syscall_64+0x73/0x220
| entry_SYSCALL_64_after_hwframe+0x42/0xb7
|
| -> #1 (&tty->termios_rwsem){++++}:
| down_write+0x39/0x50
| n_tty_flush_buffer+0x19/0xf0
| tty_buffer_flush+0x71/0x90
| tty_ldisc_flush+0x1d/0x40
| tty_port_close_start.part.5+0xa0/0x1b0
| tty_port_close+0x29/0x60
| uart_close+0x26/0x70
| tty_release+0xfc/0x4f0
| __fput+0xf1/0x200
| ____fput+0x9/0x10
| task_work_run+0x8b/0xc0
| exit_to_usermode_loop+0xbc/0xc0
| do_syscall_64+0x21b/0x220
| entry_SYSCALL_64_after_hwframe+0x42/0xb7
|
| -> #0 (&buf->lock){+.+.}:
| lock_acquire+0x95/0x240
| _mutex_lock+0x26/0x40
| flush_to_ldisc+0x1e/0xa0
| tty_flip_buffer_push+0x28/0x40
| pty_write+0x4e/0x60
| n_tty_write+0x1ae/0x480
| tty_write+0x1b3/0x320
| __vfs_write+0x35/0x160
| vfs_write+0xc1/0x1c0
| SyS_write+0x53/0xc0
| do_syscall_64+0x73/0x220
| entry_SYSCALL_64_after_hwframe+0x42/0xb7
|
| other info that might help us debug this:
|
| Chain exists of:
| &buf->lock --> &tty->termios_rwsem --> &ldata->output_lock
|
| Possible unsafe locking scenario:
|
| CPU0 CPU1
| ---- ----
| lock(&ldata->output_lock);
| lock(&tty->termios_rwsem);
| lock(&ldata->output_lock);
| lock(&buf->lock);
|
| *** DEADLOCK ***
|
| 4 locks held by sshd/3205:
| #0: (&tty->ldisc_sem){++++}, at: [< (ptrval)>] ldsem_down_read+0x2d/0x40
| #1: (&tty->atomic_write_lock){+.+.}, at: [< (ptrval)>] tty_write_lock+0x19/0x50
| #2: (&o_tty->termios_rwsem/1){++++}, at: [< (ptrval)>] n_tty_write+0x9a/0x480
| #3: (&ldata->output_lock){+.+.}, at: [< (ptrval)>] n_tty_write+0x12a/0x480
|
| stack backtrace:
| CPU: 7 PID: 3205 Comm: sshd Not tainted 4.16.18-rt9+ #187
| Call Trace:
| dump_stack+0x70/0xa7
| print_circular_bug.isra.37+0x1d8/0x1e6
| __lock_acquire+0x1284/0x1340
| ? flush_to_ldisc+0x1e/0xa0
| lock_acquire+0x95/0x240
| ? lock_acquire+0x95/0x240
| ? flush_to_ldisc+0x1e/0xa0
| _mutex_lock+0x26/0x40
| ? flush_to_ldisc+0x1e/0xa0
| flush_to_ldisc+0x1e/0xa0
| tty_flip_buffer_push+0x28/0x40
| pty_write+0x4e/0x60
| n_tty_write+0x1ae/0x480
| ? do_wait_intr_irq+0xc0/0xc0
| tty_write+0x1b3/0x320
| ? process_echoes+0x60/0x60
| __vfs_write+0x35/0x160
| ? SYSC_newfstat+0x44/0x70
| vfs_write+0xc1/0x1c0
| SyS_write+0x53/0xc0
| do_syscall_64+0x73/0x220
| entry_SYSCALL_64_after_hwframe+0x42/0xb7
| RIP: 0033:0x7f0cbc297134
| RSP: 002b:00007ffc66125a38 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
| RAX: ffffffffffffffda RBX: 0000000000000052 RCX: 00007f0cbc297134
| RDX: 0000000000000052 RSI: 00005639334fbf80 RDI: 0000000000000001
| RBP: 00005639334fbf80 R08: 0000000000000000 R09: 00007f0cbc564090
| R10: 0000000000000000 R11: 0000000000000246 R12: 00007f0cbc564760
| R13: 0000000000000052 R14: 00007f0cbc55f760 R15: 0000000000000052
Sebastian
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