From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <ee9e417a05021010353997a398@mail.gmail.com>
Date: Thu, 10 Feb 2005 13:35:46 -0500
From: Russ Cox <russcox@gmail.com>
To: 9fans <9fans@cse.psu.edu>
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Subject: [9fans] off-topic linux threading bug complaint
Topicbox-Message-UUID: 448efde4-eace-11e9-9e20-41e7f4b1d025

I'm sorry for the off-topic post, but I have no idea
where to send this and I suspect there are still Linux
experts lurking here who do.  Please feel free to forward.

	$ uname -a
	Linux t23 2.6.8-1-686 #1 Thu Oct 7 03:15:25 EDT 2004 i686 GNU/Linux
	$

This program creates a second pthread and then
lets the first pthread exit:

	$ cat x.c
	#include <stdio.h>
	#include <unistd.h>
	#include <pthread.h>

	void*
	sleeper(void *a)
	{
		for(;;)
			sleep(1000);
	}

	int
	main(void)
	{
		pthread_t id;

		pthread_create(&id, 0, sleeper, 0);
		pthread_exit(0);
	}
	$ gcc x.c -lpthread
	$ ./a.out &
	[1] 4723
	$

Because the first pthread has exited, the ps listing
makes it look like the process is a zombie, even
though it's still running:

	$ ps axwwu|grep a.out |grep -v grep
	rsc       4723  0.0  0.0     0    0 pts/2    Z    13:29   0:00
[a.out] <defunct>
	$

The -T and -L flags to ps don't help:

	$ ps -T axwwu|grep a.out |grep -v grep
	$ ps -L axwwu|grep a.out |grep -v grep
	$

It's not ps just misdisplaying information: gdb cannot attach either.

	$ gdb a.out 4723
	GNU gdb 6.1-debian
	Copyright 2004 Free Software Foundation, Inc.
	GDB is free software, covered by the GNU General Public License, and you are
	welcome to change it and/or distribute copies of it under certain conditions.
	Type "show copying" to see the conditions.
	There is absolutely no warranty for GDB.  Type "show warranty" for details.
	This GDB was configured as "i386-linux"...Using host libthread_db
library "/lib/tls/libthread_db.so.1".

	Attaching to program: /home/rsc/a.out, process 4723
	ptrace: Operation not permitted.
	/home/rsc/4723: No such file or directory.
	(gdb) quit
	$

On the other hand, if I guess the process/thread id of the
second pthread, then I can still look at that:

	$ gdb a.out 4724
	GNU gdb 6.1-debian
	Copyright 2004 Free Software Foundation, Inc.
	GDB is free software, covered by the GNU General Public License, and you are
	welcome to change it and/or distribute copies of it under certain conditions.
	Type "show copying" to see the conditions.
	There is absolutely no warranty for GDB.  Type "show warranty" for details.
	This GDB was configured as "i386-linux"...Using host libthread_db
library "/lib/tls/libthread_db.so.1".

	Attaching to program: /home/rsc/a.out, process 4724
	Reading symbols from /lib/tls/libpthread.so.0...done.
	[Thread debugging using libthread_db enabled]
	[New Thread 1083603888 (LWP 4724)]
	Loaded symbols for /lib/tls/libpthread.so.0
	Reading symbols from /lib/tls/libc.so.6...done.
	Loaded symbols for /lib/tls/libc.so.6
	Reading symbols from /lib/ld-linux.so.2...done.
	Loaded symbols for /lib/ld-linux.so.2
	Reading symbols from /lib/libgcc_s.so.1...done.
	Loaded symbols for /lib/libgcc_s.so.1
	0x400dc5dc in nanosleep () from /lib/tls/libc.so.6
	(gdb) quit
	The program is running.  Quit anyway (and detach it)? (y or n) y
	Detaching from program: /home/rsc/a.out, process 4724
	$

But that's stupid.  The process known as 4723 has not
exited, and it should be debuggable, even though the
pthread known as 4723 _has_ exited.

Russ