linux-kernel - orphan process is causing deadlock in __kernel

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Date:	Wed, 25 Jul 2012 17:14:28 -0400
From:	Sri Ram Vemulpali <sri.ram.gmu06@...il.com>
To:	linux-kernel-mail <linux-kernel@...r.kernel.org>,
	linux-newbie@...r.kernel.org, linux-netdev <netdev@...r.kernel.org>
Subject: orphan process is causing deadlock in __kernel_vsyscall ()

Hello All,

I am debugging a problem which I do not know how to solve. I know what
is happening, but cannot find solution to it. If anyone has any
suggestions or solution that would be really grateful and I appreciate
it.

Here is the description.

I have a thread which spawns a child process using fork() and in child
context it execv to shell and parent thread waits on waitpid(). This
child process is nothing but a shell /bin/sh, so that from program I
can create shell and run some commands.
In this shell instance, I run tshark for capturing network traffic. I
have really high traffic going, so system is really busy. All is good
so far, but when I send SIGKILL to child process (shell) from another
thread from same application,
child process (shell) gets terminated and tshark is assigned to
init(1) process as orphan. The parent thread which spawned the child
process unblocks from waitpid() after cleaning child instance and
continues it execution.

Here is the problem:
Once the parent thread resumes from waitpid(), it hits fprintf to
print message "exiting from shell" to console. In here the main thread
gets blocked in __kernel_vsyscall () entered from fprintf. Following
is my stack trace from gdb.
But after some more debugging and found that, if I kill from another
shell "tshark" process which is assigned to init(1) as child, the
parent thread unblocks from __kernel_vsyscall ().

Can anyone tell what is going on, is this related to futex spinning
problem where lock is been held by tshark. How would I approach this
problem?
Or is there a way to kill all child process of a process without
making them orphan. Because when a child process is running at the
time parent is exiting, that child process is assigned to init(1) as
orphan process.
Can this behavior be modified. Please let me know any suggestions and
thoughts. Thanks in advance.

-- 
Regards,
Sri.

(gdb) backtrace
#0  0xffffe424 in __kernel_vsyscall ()
#1  0xb7e6396b in write () from /lib/libc.so.6
#2  0xb7e0c2ef in _IO_new_file_write () from /lib/libc.so.6
#3  0xb7e0bf93 in new_do_write () from /lib/libc.so.6
#4  0xb7e0c296 in _IO_new_do_write () from /lib/libc.so.6
#5  0xb7e0ce20 in _IO_new_file_overflow () from /lib/libc.so.6
#6  0xb7e0c0cd in _IO_new_file_xsputn () from /lib/libc.so.6
#7  0xb7de3634 in vfprintf () from /lib/libc.so.6
#8  0x09ad62a2 in acme_fprintf (stream=0xb7ee2560,
    format=0x9f17694 "shell exited. returning to acli\n")
    at /home/svemulpali/cc/svemulpali_SCOTTY_main/linux/private/common/src/coutOverride.cpp:42
#9  0x094a9345 in vxShellCmdStartWait (syscmd=0x0)
    at /home/svemulpali/cc/svemulpali_SCOTTY_main/linux/private/lib/stub/vxShellLib.cpp:210
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