linux-kernel - Re: [PATCH 08/11] ptrace: move fallback JOBCTL_TRAPPING clearing to get_signal_to

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Date:	Thu, 12 May 2011 17:40:19 +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
Subject: Re: [PATCH 08/11] ptrace: move fallback JOBCTL_TRAPPING clearing
	to get_signal_to_deliver()

On 05/11, Tejun Heo wrote:
>
> On Wed, May 11, 2011 at 05:48:34PM +0200, Oleg Nesterov wrote:
> > On 05/08, Tejun Heo wrote:
> > > Move the fallback clearing to the end of get_signal_to_deliver() so
> > > that TRAPPING is maintained while tracee is inside signal delivery
> > > path.  When killed, tracee is guaranteed to leave signal delivery path
> > > in finite amount of time and thus TRAPPING is still guaranteed to be
> > > cleared on kill.
> >
> > Mostly yes, but we can race with freeze_processes() and deadlock.
>
> Ah... the try_to_freeze().  Nice spotting.  It isn't necessarily a
> deadlock tho.

Yes, it is not "fatal" for the system.

> I
> think the right thing to do here is making the TRAPPING sleep an
> interruptible one and let the syscall restart logic deal with it.

Oh, yes, we can. But this adds more complications again. We can fix
this particular problem simpler. Say, freezer_do_not_count(). There
are other options afaics.

> * Make TRAPPING wait INTERRUPTIBLE.

I am starting to strongly dislike TRAPPING outside of do_signal_stop().

> * Move clearing of pending group stop and traps to the actions which
>   require such clearing.

Not sure this is enough. OTOH, perhaps this is unnneeded.

I'll write another email to explain what I mean.

Oleg.

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