Re: [PATCH 2/7] freezer: add missing mb's to freezer_count() and freezer_should_skip()

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

Hello, Oleg.

On Mon, Oct 22, 2012 at 07:44:04PM +0200, Oleg Nesterov wrote:
> >  static inline void freezer_count(void)
> >  {
> >  	current->flags &= ~PF_FREEZER_SKIP;
> > +	/*
> > +	 * If freezing is in progress, the following paired with smp_mb()
> > +	 * in freezer_should_skip() ensures that either we see %true
> > +	 * freezing() or freezer_should_skip() sees !PF_FREEZER_SKIP.
> > +	 */
> > +	smp_mb();
> >  	try_to_freeze();
> 
> I agree, this looks like a bug fix.

Yeah, and this isn't dangerous at all.  I'll ping -stable.

> > -static inline int freezer_should_skip(struct task_struct *p)
> > +static inline bool freezer_should_skip(struct task_struct *p)
> >  {
> > -	return !!(p->flags & PF_FREEZER_SKIP);
> > +	/*
> > +	 * The following smp_mb() paired with the one in freezer_count()
> > +	 * ensures that either freezer_count() sees %true freezing() or we
> > +	 * see cleared %PF_FREEZER_SKIP and return %false.  This makes it
> > +	 * impossible for a task to slip frozen state testing after
> > +	 * clearing %PF_FREEZER_SKIP.
> > +	 */
> > +	smp_mb();
> > +	return p->flags & PF_FREEZER_SKIP;
> >  }
> 
> I am not sure we really need smp_mb() here. Speaking of cgroup_freezer,
> it seems that a single mb() after "->state = CGROUP_FREEZING" should be
> enough.

Hmmm... I agree pairing there would work too.

> But even if I am right, I agree that it looks better in freezer_should_skip()
> and this is more robust.

But, yeah, performance implications at this level are almost
completely irrelavent here and I think pairing freezer_should_skip()
is easier to read.

> So I think the patch is fine and fixes the bug.

Awesome.

> We probably have another similar race. If ptrace_stop()->may_ptrace_stop()
> returns false, the task does
> 
> 	__set_current_state(TASK_RUNNING);
> 	// no mb in between
> 	try_to_freeze();
> 
> And this can race with task_is_stopped_or_traced() check in the same way.
> (of course this is only theoretical).
> 
> do_signal_stop() is probably fine, we can rely on ->siglock.

Hmm....  Guess we should drop __ from set_current_state.  I wonder
whether we should just add mb to freezing()?  What do you think?

Thanks.

--
tejun