possible recursive locking detected n_tty_write vs. process_echoes

[Andreas Starzer]

With enabled deadlock detection the following warning occurs when
connected via ssh (Dropbear server v2015.67).
With "while :; do dmesg -c; done" the problem was fast reproduceable.

System: Kernel with RT-patch applied:
Linux version 3.10.63-rt65 (gcc version 4.9.2 (iMX6) ) #1 SMP PREEMPT RT

This seems to influence the RT-behavior somehow!!!
I made a 1ms nano-sleep timer with jitter observation and the jitter
is quite huge until this warning occured.

After this warning showed up once the jitter is gone!!!

Without Full-Preemption the warning does not show up. (Tried: Desktop
/ Low-Latency desktop / Basic RT)

Please help!

CPU: Freescale i.MX6 Quad
The jitter observer is a simple kthread with prio 60 (so above IRQs)
which does hrtimer_nanosleep for 1ms and measures the elapsed time
with getrawmonotonic.


[  416.153094]
[  416.153097] =============================================
[  416.153099] [ INFO: possible recursive locking detected ]
[  416.153106] 3.10.63-rt65-svn68 #1 Tainted: G           O
[  416.153108] ---------------------------------------------
[  416.153113] dropbear/306 is trying to acquire lock:
[  416.153151]  (&ldata->output_lock){+.+...}, at: [<802bc728>]
process_echoes+0x48/0x2b4
[  416.153154]
[  416.153154] but task is already holding lock:
[  416.153168]  (&ldata->output_lock){+.+...}, at: [<802bce2c>]
n_tty_write+0x148/0x464
[  416.153169]
[  416.153169] other info that might help us debug this:
[  416.153172]  Possible unsafe locking scenario:
[  416.153172]
[  416.153174]        CPU0
[  416.153176]        ----
[  416.153180]   lock(&ldata->output_lock);
[  416.153185]   lock(&ldata->output_lock);
[  416.153187]
[  416.153187]  *** DEADLOCK ***
[  416.153187]
[  416.153189]  May be due to missing lock nesting notation
[  416.153189]
[  416.153193] 2 locks held by dropbear/306:
[  416.153209]  #0:  (&tty->atomic_write_lock){+.+...}, at:
[<802b9b04>] tty_write_lock+0x1c/0x5c
[  416.153223]  #1:  (&ldata->output_lock){+.+...}, at: [<802bce2c>]
n_tty_write+0x148/0x464
[  416.153225]
[  416.153225] stack backtrace:
[  416.153232] CPU: 3 PID: 306 Comm: dropbear Tainted: G           O
3.10.63-rt65-svn68 #1
[  416.153237] Backtrace:
[  416.153261] [<80011a60>] (dump_backtrace+0x0/0x108) from
[<80011c70>] (show_stack+0x18/0x1c)
[  416.153273]  r6:809dac30 r5:808b7944 r4:809dac30 r3:00000000
[  416.153290] [<80011c58>] (show_stack+0x0/0x1c) from [<80615888>]
(dump_stack+0x24/0x28)
[  416.153313] [<80615864>] (dump_stack+0x0/0x28) from [<80071c88>]
(__lock_acquire+0x1d04/0x2018)
[  416.153325] [<8006ff84>] (__lock_acquire+0x0/0x2018) from
[<80072790>] (lock_acquire+0x68/0x7c)
[  416.153340] [<80072728>] (lock_acquire+0x0/0x7c) from [<80618e74>]
(_mutex_lock+0x38/0x48)
[  416.153350]  r7:8c9d1000 r6:8c9d0000 r5:8cd5d58e r4:8c9d12a8
[  416.153364] [<80618e3c>] (_mutex_lock+0x0/0x48) from [<802bc728>]
(process_echoes+0x48/0x2b4)
[  416.153368]  r4:8c9d12f8
[  416.153381] [<802bc6e0>] (process_echoes+0x0/0x2b4) from
[<802beb1c>] (n_tty_receive_buf+0x1040/0x1044)
[  416.153396] [<802bdadc>] (n_tty_receive_buf+0x0/0x1044) from
[<802c26e4>] (flush_to_ldisc+0x11c/0x16c)
[  416.153407] [<802c25c8>] (flush_to_ldisc+0x0/0x16c) from
[<802c2778>] (tty_flip_buffer_push+0x44/0x48)
[  416.153419] [<802c2734>] (tty_flip_buffer_push+0x0/0x48) from
[<802c38a0>] (pty_write+0x5c/0x6c)
[  416.153425]  r5:8c573800 r4:00000001
[  416.153436] [<802c3844>] (pty_write+0x0/0x6c) from [<802bce44>]
(n_tty_write+0x160/0x464)
[  416.153446]  r6:8c573800 r5:8c9d1400 r4:00000001 r3:802c3844
[  416.153459] [<802bcce4>] (n_tty_write+0x0/0x464) from [<802b9c60>]
(tty_write+0x11c/0x2e0)
[  416.153475] [<802b9b44>] (tty_write+0x0/0x2e0) from [<800d1e10>]
(vfs_write+0xb8/0x194)
[  416.153484] [<800d1d58>] (vfs_write+0x0/0x194) from [<800d2394>]
(SyS_write+0x44/0x80)
[  416.153498]  r9:00000000 r8:00000000 r7:013002cc r6:00000000 r5:00000001
[  416.153498] r4:8c6cf540
[  416.153511] [<800d2350>] (SyS_write+0x0/0x80) from [<8000e580>]
(ret_fast_syscall+0x0/0x48)
[  416.153524]  r9:8c9a8000 r8:8000e744 r7:00000004 r6:012f9128 r5:00000001
[  416.153524] r4:012f9668
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