linux-kernel - Re: [PATCH 08/10] ptrace: participate in group stop from ptrace_stop() iff the task is trapping for group stop

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Message-Id: <20110128213009.340C7183C1E@magilla.sf.frob.com>
Date:	Fri, 28 Jan 2011 13:30:09 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	oleg@...hat.com, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 08/10] ptrace: participate in group stop from ptrace_stop() iff the task is trapping for group stop

> A visible behavior change is increased likelihood of delayed group
> stop completion if the thread group contains one or more ptraced
> tasks.

I object to that difference in behavior.  As I've said before, I don't
think there should be any option to circumvent a group stop via ptrace.
If you think otherwise, you have a hard road to convince me of it.  

It was always the intent that traced tasks should participate in the
group stop bookkeeping.  I suspect the better line of fixes will be just
to tie up the loose ends of the ptrace interactions so that all ptrace
stops do the correct group_stop_count bookkeeping and notifications.  If
there is a group stop in progress but not yet complete, then PTRACE_CONT
on a thread in the group should probably just move it from TASK_TRACED
to TASK_STOPPED without resuming it at all.  

Once a group stop is complete, then probably the ideal is that
PTRACE_CONT would not resume a thread until a real SIGCONT cleared the
job control stop condition.  But it's likely that existing ptrace users
have expectations contrary to that, so we'll have to see.

Thanks,
Roland
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