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Subject: Re: [PATCH 3/8] vfork: make it killable
From: Matt Fleming <matt.fleming@linux.intel.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Roland McGrath <roland@hack.frob.com>, Tejun Heo <tj@kernel.org>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org
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Date: Fri, 29 Jul 2011 16:32:45 +0100
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On Fri, 2011-07-29 at 16:32 +0200, Oleg Nesterov wrote:
> On 07/29, Matt Fleming wrote:
> >
> > On Wed, 2011-07-27 at 18:32 +0200, Oleg Nesterov wrote:
> >
> > [...]
> >
> > >  static long clone_vfork_finish(struct task_struct *child,
> > >  				struct completion *vfork_done, long pid)
> > >  {
> > > -	freezer_do_not_count();
> > > -	wait_for_completion(vfork_done);
> > > -	freezer_count();
> > > +	int killed = wait_for_completion_killable(vfork_done);
> > > +
> > > +	if (killed) {
> > > +		struct completion *steal = xchg(&child->vfork_done, NULL);
> > > +		/* if we race with complete_vfork_done() we have to wait */
> > > +		if (unlikely(!steal))
> > > +			wait_for_completion(vfork_done);
> > > +
> > > +		return -EINTR;
> > > +	}
> >
> > Hmm.. isn't this inherently racy anyway? Why go to the trouble of trying
> > to handle this race at all?
> 
> Suppose the child does xchg() and sees vfork_done != NULL. In this
> case the parent shouldn't return from do_fork() until the child
> does complete(), this "struct completion" was allocated on parent's
> stack.
> 
> OK, I am starting to agree this looks overcomplicated, task_lock()
> can make the code look simpler (see 0/8).

Yeah, I think the code in 0/8 looks like a better solution.