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
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