Re: [PATCH 3/8] vfork: make it killable

Oleg Nesterov
2011-07-29T14:35:31Z

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).

Oleg.