linux-kernel - Re: [PATCH 6/9] job control: make task_clear_jobctl

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Message-ID: <20110516160019.GB15918@redhat.com>
Date:	Mon, 16 May 2011 18:00:19 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jan.kratochvil@...hat.com, vda.linux@...glemail.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu, bdonlan@...il.com
Subject: Re: [PATCH 6/9] job control: make task_clear_jobctl_pending()
	clear TRAPPING automatically

On 05/16, Tejun Heo wrote:
>
> Hello,
>
> On Mon, May 16, 2011 at 02:25:35PM +0200, Oleg Nesterov wrote:
> > On 05/13, Tejun Heo wrote:
> > >
> > > @@ -264,6 +267,9 @@ void task_clear_jobctl_pending(struct task_struct *task, unsigned int mask)
> > >  		mask |= JOBCTL_STOP_CONSUME | JOBCTL_STOP_DEQUEUED;
> > >
> > >  	task->jobctl &= ~mask;
> > > +
> > > +	if (!(task->jobctl & JOBCTL_PENDING_MASK))
> > > +		task_clear_jobctl_trapping(task);
> > >  }
> >
> > So, SIGCONT clears JOBCTL_TRAPPING and wakes up the tracer.
>
> If JOBCTL_TRAPPING is set && JOBCTL_STOP_PENDING was the only pending
> condition.
>
> > I can't really understand this without seeing the next changes, but
> > it seems this makes some things worse, although I am not sure.
>
> It's a safety mechanism.  We shouldn't have TRAPPING set when no
> stop/trap is pending and the above establishes that invariant

Hmm. I thought that SIGCONT should add the new TRAPPING... My head spins.

> > For example. PTRACE_SEIZE should guarantee the tracee will trap and
> > report. However, if the tracee is stopped during attach, we can race
> > with SIGCONT. The previous version had the similar problem afaics, but
> > it was easy (I think) to fix. Now that SIGCONT clears JOBCTL_TRAPPING
> > we need more complications.
>
> This problem doesn't exist anymore.

OK. Of course I do not understand your explanation right now, I do not
see the code, but I trust you ;)


My only point, I still think that it is better to not apply these
preparations right now, without the next SEIZE/etc changes.

Oleg.

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