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Message-ID: <CAOihqkJpbeYCEBiOt=Ms55tVWLLONvJqqr8Y6OxG1HwfYL6ZFg@mail.gmail.com>
Date: Fri, 6 Feb 2015 13:03:06 +0100
From: Andreas Starzer <stzlinux@...il.com>
To: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, mpatocka@...hat.com, peter@...leysoftware.com,
peterz@...radead.org, mika.westerberg@...ux.intel.com
Subject: possible recursive locking detected n_tty_write vs. process_echoes
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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