linux-kernel - Re: [PATCH 1/8] job control: Don't set group_stop exit

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Date:	Wed, 23 Mar 2011 17:40:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	roland@...hat.com, jan.kratochvil@...hat.com,
	vda.linux@...glemail.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	indan@....nu
Subject: Re: [PATCH 1/8] job control: Don't set group_stop exit_code if
	re-entering job control stop

On 03/23, Tejun Heo wrote:
>
> Hello,
>
> On Tue, Mar 22, 2011 at 07:44:15PM +0100, Oleg Nesterov wrote:
> > Suppose that debugger PTRACE_CONTs the stopped thread and then it
> > gets SIGSTOP and calls do_signal_stop() again. In theory this all is
> > possible before SIGNAL_STOP_STOPPED. This can confuse real_parent.
> > Say, real_parent itself sends SIGTTIN to the child and naturally
> > expects WIFSTOPPED() == SIGTTIN.
>
> Hmmm... There are two competing signals in that case - SIGTTIN sent by
> the parent and SIGSTOP sent by someone else.

"someone else" can be PTRACE_CONT(SIGSTOP) from the debugger.

> I can't think of a scenario where the parent would be able to
> differentiate the two signals (in the sense that it can say the latter
> is the wrong signal).  If you can, please share.

I didn't mean this is really wrong or can lead to some problems.
Just this looks inconsistent a bit.

> > Not sure I understand... We are setting GROUP_STOP_PENDING | CONSUME
> > again. T2 has already reported ptrace_stop(CLD_STOPPED) to the tracer.
> > It is stopped. Now it will report another CLD_STOPPED after PTRACE_CONT.
>
> Okay, I see.  Maybe we should discern between traced for group stop
> from other traps but then again given the group stop re-entering while
> ptraced it can be considered a relatively consistent behavior.  Yeah,
> but probably better to remove the double reporting.

Yes. I have a vague feeling a new GROUP_STOP_YES_I_AM_STOPPED can
be useful anyway. It should be set by task_participate_group_stop()
if the task participates. We will see.

Oleg.

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