linux-kernel - Re: Strange stop-signal behavior in multithreaded program with defunct main

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Date:	Thu, 30 Oct 2008 10:55:40 -0500
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Oleg Nesterov" <oleg@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>,
	"Bert Wesarg" <bert.wesarg@...glemail.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Roland McGrath" <roland@...hat.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: Re: Strange stop-signal behavior in multithreaded program with defunct main

Hi Oleg,

On Thu, Oct 30, 2008 at 6:00 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 10/28, Michael Kerrisk wrote:
>>
>> Bert Wesarg described a scenario that I quickly replicated on
>> 2.6.28-rc2 (and 2.6.25 -- it's not a regression in 2.6.28-rc)
>> using the program below: if we have a multithreaded process
>> with a defunct main thread running on a tty, and that
>> process is sent a stop signal (either ^Z (SIGTSTP) or a stop
>> signal sent from another terminal using kill(1)), then:
>>
>> a) the terminal is locked up; and
>>
>> b) the program is unresponsive to any other signal, except SIGKILL
>> or SIGCONT.
>
> Yes, known problem. Please look at
>
>        [RFC,PATCH 3/3] do_wait: fix waiting for stopped group with dead leader
>        http://marc.info/?t=119713920000003

Okay -- thanks for the info.  I've added some text to man-pages to
cover this bug.

Cheers,

Michael

--- a/man3/pthread_exit.3
+++ b/man3/pthread_exit.3
@@ -21,7 +21,7 @@
 .\" Formatted or processed versions of this manual, if unaccompanied by
 .\" the source, must acknowledge the copyright and authors of this work.
 .\"
-.TH PTHREAD_EXIT 3 2008-10-24 "Linux" "Linux Programmer's Manual"
+.TH PTHREAD_EXIT 3 2008-10-30 "Linux" "Linux Programmer's Manual"
 .SH NAME
 pthread_exit \- terminate calling thread
 .SH SYNOPSIS
@@ -87,6 +87,18 @@ The value pointed to by
 .IR retval
 should not be located on the calling thread's stack,
 since the contents of that stack are undefined after the thread terminates.
+.SH BUGS
+Currently,
+.\" Linux 2.6.27
+there are limitations in the kernel implementation logic for
+.BR wait (2)ing
+on a stopped thread group with a dead thread group leader.
+This can manifest in problems such as a locked terminal if a stop signal is
+sent to a foreground process whose thread group leader has already called
+.BR pthread_exit (3).
+.\" FIXME . review a later kernel to see if this gets fixed
+.\" http://thread.gmane.org/gmane.linux.kernel/611611
+.\" http://marc.info/?l=linux-kernel&m=122525468300823&w=2
 .SH SEE ALSO
 .BR pthread_create (3),
 .BR pthread_join (3),
--
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