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

From: Roland McGrath
Date: Fri Jan 28 2011 - 16:30:59 EST

> 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.

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