linux-kernel - Re: n_tty_write() going into schedule but NOT coming out

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Date:	Tue, 2 Apr 2013 16:39:54 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	lkml <linux-kernel@...r.kernel.org>,
	<linux-serial@...r.kernel.org>, "Ingo Molnar" <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: n_tty_write() going into schedule but NOT coming out

Hi Peter,

On 04/01/2013 08:40 PM, Peter Hurley wrote:
> On Mon, 2013-04-01 at 19:27 +0530, Vineet Gupta wrote:
>> Hi,
>>
>> Any thoughts: I observe the same issue even with CONFIG_PREEMPT and
>> CONFIG_PREEMPT_COUNT
>>
>> -Vineet
>>
>> On 03/30/2013 06:05 PM, Vineet Gupta wrote:
>>> Hi,
>>>
>>> I've been stress testing ARC Linux 3.8 (same happens for 3.9-rc3 as well). The
>>> setup has 3 telnet sessions, each running find . -name "*" in a loop.
>>> The platform is a FPGA @ 80 MHz, running a single core ARC700 so kernel .config
>>> has !SMP and PREEMPT_NONE.
>>>
>>> After ~10 mins of run, I see that one of the telnet session gets stuck (and later
>>> the 2nd one as well), while system is still alive, 3rd telnet is running find merrily.
>>>
>>> [ARCLinux]$ ps
>>> ....
>>>     7 root       0:00 inetd
>>>    62 root       0:00 -/bin/sh
>>>    64 root       1:34 telnetd -i -l /bin/sh
>>>    65 root       0:00 /bin/sh
>>>    75 root       1:47 telnetd -i -l /bin/sh
>>>    76 root       0:00 /bin/sh
>>>    79 root       0:53 telnetd -i -l /bin/sh
>>>    80 root       0:00 /bin/sh
>>>   281 root       0:00 find / -name *	<--- stuck
>>>   358 root       0:03 find / -name *	<--- stuck
>>>   377 root       0:00 find / -name *
>>>   378 root       0:00 ps
>>>
>>> Hung find task is sitting in the schedule() call in n_tty_write()
>>>
>>> [ARCLinux]$ cat /proc/281/stack
>>> [<8065945e>] n_tty_write+0x23a/0x424
>>> [<80655cd4>] tty_write+0x1ac/0x2d4
>>> [<805976ba>] vfs_write+0x92/0x110
>>> [<80597816>] sys_write+0x4e/0x88
>>> [<8050e780>] ret_from_system_call+0x0/0x4
> Likely the writer is stuck because the receive buffer is full and the
> reader is hung. What are the respective shells and telnetd doing?

telnetd is in select syscall - waiting for new connections ?
shell is wait(2)ing - likely on the child "find" task.

/ # cat /proc/62/stack
[<8059e26a>] do_select+0x4b6/0x59c
[<8059e4f8>] core_sys_select+0x1a8/0x690
[<8059ea74>] sys_pselect6+0x94/0x254
[<80505fb4>] ret_from_system_call+0x0/0x4

/ # cat /proc/63/stack
[<80513a4a>] do_wait+0x166/0x188
[<80513ace>] sys_wait4+0x62/0xac
[<80505fb4>] ret_from_system_call+0x0/0x4

/ # cat /proc/445/stack
[<8064c7f2>] n_tty_write+0x23a/0x424
[<80649038>] tty_write+0x1a0/0x2b4
[<8058a4b6>] vfs_write+0x92/0x110
[<8058a612>] sys_write+0x4e/0x88
[<80505fb4>] ret_from_system_call+0x0/0x4


> PS - include the controlling tty column in your ps output :)

S   UID   PID  PPID   VSZ   RSS TTY   STIME TIME     CMD
S     0    62    47  1768   488 0:0   16:01 00:01:44 telnetd -i -l /bin/sh
S     0    63    62  1768   512 pts2  16:01 00:00:00 /bin/sh
S     0   445    63  1840   488 pts2  16:30 00:00:00 find / -name *

Thx,
-Vineet
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