linux-kernel - Re: Hanging threads with pthread

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Message-Id: <201108191128.09930.pedro@codesourcery.com>
Date:	Fri, 19 Aug 2011 11:28:09 +0100
From:	Pedro Alves <pedro@...esourcery.com>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	Philipp Marek <philipp.marek@...bit.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Hanging threads with pthread_detach and gdb

On Friday 19 August 2011 09:54:43, Lin Ming wrote:
> =====
> gdb wait for pid=2431
> gdb is going to sleep ...
> gdb children list:
>     pid=2431, exit_state=16, exit_signal=17
> gdb ptrace list:
>     pid=2436, exit_state=16, exit_signal=-1
>     pid=2431, exit_state=16, exit_signal=17
> =====
> 
> exit_state 16 is EXIT_ZOMBIE state.
> pid 2431 is the thread group leader.
> pid 2436 is the thread group member(other members have been removed).
> 
> gdb is waiting the group leader, but it fails because the group is not
> empty. Then gdb goes to sleep.
> 
> At this moment, all threads have gone into EXIT_ZOMBIE state.
> So no thread can wake up gdb anymore.
> 
> That's why gdb hangs.
> 
> I'm not familiar with pthread semantics.
> Is this a problem need to be fixed?
> 

Yeah, it's an ugly ptrace quirk.  waitpid'ing the exited
tg leader hangs if there are other threads in the group,
even if they're zombies.

I believe this is fixed / worked around mainline GDB:

 http://sourceware.org/bugzilla/show_bug.cgi?id=10970
 http://sourceware.org/bugzilla/show_bug.cgi?id=12702
 http://sourceware.org/ml/gdb-patches/2011-05/msg00401.html

Though it'd be nice not to need the workaround...

-- 
Pedro Alves
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