linux-kernel - Re: [PATCH 02/10] job control: introduce JOBCTL_TRAP

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Date:	Thu, 19 May 2011 18:19:23 +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 02/10] job control: introduce JOBCTL_TRAP_STOP and use
	it for group stop trap

On 05/19, Tejun Heo wrote:
>
> On Wed, May 18, 2011 at 06:48:14PM +0200, Oleg Nesterov wrote:
> > > Although this adds an unlock-relocking between checking of
> > > JOBCTL_STOP_PENDING and actually trapping for STOP, this doesn't
> > > affect correctness.
> >
> > Well, I think it does affect. Although the problem is minor.
> >
> > > ptrace_stop() already had conditional unlock-relocking
> >
> > Yes, but ptrace_stop() can't send the CLD_STOPPED notfication before
> > the tracee is ready for do_wait(WNOHANG). Contrary, get_signal_to_deliver()
> > can if we race with SIGCONT.
>
> That CLD_STOPPED notification is actually for continuation (although
> it's indistinguishible from actual stopped notification) and as such
> the ptracer has to query the tracee state after the notification
> signal and can't expect it to be in TRACED.  Please consider the
> following scenario.

OK, agreed.

> > Hmm. And afaics there is a bug in do_signal_stop(), after
> > "[PATCH 6/9] job control: make task_clear_jobctl_pending() clear TRAPPING automatically"
> > ->jobctl &= ~JOBCTL_STOP_SIGMASK is no longer safe. We can clear _TRAPPING
> > without wakeup.
>
> Hmmm?  ->jobctl &= ~JOBCTL_STOP_SIGMASK clears lower sixteen bits.
> All the flags including TRAPPING live in the upper sixteen bits.

Indeed, I was confused. Thanks for correcting me.

Oleg.

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