Date: Wed, 22 Dec 2010 17:00:16 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Tejun Heo <tj@kernel.org>
Cc: roland@redhat.com, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org, rjw@sisk.pl,
        jan.kratochvil@redhat.com
Subject: Re: [PATCH 10/16] ptrace: clean transitions between TASK_STOPPED
	and TRACED
Message-ID: <20101222160016.GA11722@redhat.com>
References: <1291654624-6230-1-git-send-email-tj@kernel.org> <1291654624-6230-11-git-send-email-tj@kernel.org> <20101220150037.GE11583@redhat.com> <20101221173155.GE13285@htj.dyndns.org> <20101222113948.GA30266@redhat.com> <20101222151429.GC8061@htj.dyndns.org>
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On 12/22, Tejun Heo wrote:
>
> On Wed, Dec 22, 2010 at 12:39:48PM +0100, Oleg Nesterov wrote:
> >
> > sys_ptrace() only works for the single thread who did PTRACE_ATTACH,
> > but do_wait() should work for its sub-threads.
> >
> > 	1. the tracer knows that the tracee is stopped
> >
> > 	2. the tracer does ptrace(ATTACH)
> >
> > 	3. the tracer's sub-thread does do_wait()
> >
> > Note! Personally I think we can ignore this "problem", I do not
> > think it can break anything except some specialized test-case.
>
> But if ptrace(ATTACH) doesn't return until the transition is complete
> when the task is already stopped, the tracer's sub-thread's do_wait()
> will behave exactly the same.  The only difference would be that
> ptrace(ATTACH) may now block and/or is failed by a signal delivery.
>
> How would #3 behave differently if STOPPED -> TRACED transition is
> guaranteed to be complete by the end of #2?

Ahhh, sorry. I meant, two threads can do 2. and 3. at the same time.

But let me repeat, it is not that I think we should worry. I mentioned
this only because I think it is better to discuss everything we can,
even the really minor things.

Oleg.

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