linux-kernel - Re: [PATCH 4/9] ptrace: relocate set_current_state(TASK_TRACED) in ptrace

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Message-ID: <20110516131608.GX23665@htj.dyndns.org>
Date:	Mon, 16 May 2011 15:16:08 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 4/9] ptrace: relocate set_current_state(TASK_TRACED) in
 ptrace_stop()

Hey, Oleg.

On Mon, May 16, 2011 at 01:57:11PM +0200, Oleg Nesterov wrote:
> > and helps future updates to group stop participation.
> 
> OK, so I assume we need this change.

We don't necessarily need it but it makes things prettier later.

> But the comment looks a bit confusing to me. This is fine, I almost never
> read them ;) Just I'd like to ensure I din't miss something.

Oleg, IIRC, those comments were taken from your email pointing out
that set_current_state() needs to happen before clearing of TRAPPING,
so, if you're confused, I'm confused too. :-)

> > +	 * We're committing to trapping.  TRACED should be visible before
> > +	 * TRAPPING is cleared
> 
> This looks as if you explain the barrier in set_current_state(). And,
> btw, why can't we use __set_current_state() here ?
> 
> And. not only TRACED, at least ->exit_code should be visible as well.

The racy part was task_is_stopped_or_traced() in task_stopped_code()
and the value of exit_code doesn't matter at that point.  So, we need
at least smp_wmb() between __set_current_state() and clearing
TRAPPING.

> IOW. It is not that TRACED should be visible before jobctl &= ~JOBCTL_TRAPPING,
> we should correctly update the tracee before __wake_up_sync_key(), and I assume
> this is what the comment says.
> 
> Correct?

All we need to update on the tracee is tracee->state and
~JOBCTL_TRAPPING and __wake_up_sync_key() can be considered single
operation.  One doesn't make sense with the other.  Anyways, if you
wanna update the comment, please go ahead.

Thanks.

-- 
tejun
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