linux-kernel - Re: [PATCH v2 0/3] make vfork killable

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Message-ID: <20110823220158.GA28444@count0.beaverton.ibm.com>
Date:	Tue, 23 Aug 2011 15:01:58 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...k.frob.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Matt Fleming <matt.fleming@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH v2 0/3] make vfork killable

On Sat, Aug 13, 2011 at 06:18:14PM +0200, Tejun Heo wrote:
> Hello, Oleg.
> 
> On Fri, Aug 12, 2011 at 07:55:50PM +0200, Oleg Nesterov wrote:
> > > an alternative approach
> > > could be handling vfork waiting as a type of job control stop.
> > 
> > Well, I didn't see the code, but to be honest this doesn't look
> > like a good idea to me. Firstly, personally I do not think this
> > has something to do with the job control stop.
> > 
> > And, to me sys_restart_syscall() looks like the very natural
> > approach, and simple.
> 
> I've been playing with this and it does a bit further than
> implementation simplicity.  Currently, we have three different modes
> of stopping a task.
> 
> * Regular job control and ptrace.
> * vfork wait.
> * cgroup freeze.
> 
> Currently, all three behave differently and the latter two use
> UNINTERRUPTIBLE sleep causing rather nasty problems.  If we want to
> fix the UNINTERRUPTIBLE sleep problem, we end up introducing a new
> user visible state no matter which way we go - ie. a task will be in a
> state which isn't UNINTERRUPTIBLE sleep but still behave differently
> in terms of signal delivery and job control.
> 
> What's needed is this different state of being stopped which reponds
> to all kernel's desires (killing and ptracing) but stays stopped
> regardless of what the user requests.
> 
> There's multiple ways to implement this and forced syscall restart is
> one way to achieve it - ie. while the stop condition is pending,
> syscall is forced to be restarted after interruption and re-enter
> stop.  The downside is that that wouldn't work with cgroup freeze at
> all - there's no syscall to restart.
> 
> So, what I'm proposing is to basically add another job control state
> which is similar to process group stop but controlled by other
> parameters like vfork wait condition or control group frozen state.

Are you proposing one new jobctl state for vfork and another for
the cgroup freezer?

> This allows these stops to be handled in a way very similar to already
> esablished job control states including interaction with ptrace.

Interesting. So long as shells and ptracers in containers would not see
jobctl traps/events triggered by tasks outside the container it sounds
fine.

Cheers,
	-Matt Helsley
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