Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert_state

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Wed, Jun 17, 2015 at 2:41 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > * Andy Lutomirski <luto@kernel.org> wrote:
> >
> >> This will let us sprinkle sanity checks around the kernel without
> >> making too much of a mess.
> >>
> >> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> >> ---
> >>  include/linux/context_tracking.h | 8 ++++++++
> >>  1 file changed, 8 insertions(+)
> >>
> >> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> >> index 2821838256b4..0fbea4b152e1 100644
> >> --- a/include/linux/context_tracking.h
> >> +++ b/include/linux/context_tracking.h
> >> @@ -57,6 +57,13 @@ static inline void context_tracking_task_switch(struct task_struct *prev,
> >>       if (context_tracking_is_enabled())
> >>               __context_tracking_task_switch(prev, next);
> >>  }
> >> +
> >> +static inline void context_tracking_assert_state(enum ctx_state state)
> >> +{
> >> +     rcu_lockdep_assert(!context_tracking_is_enabled() ||
> >> +                        this_cpu_read(context_tracking.state) == state,
> >> +                        "context tracking state was wrong");
> >> +}
> >
> > Please don't introduce assert() style debug check interfaces!
> >
> > (And RCU should be fixed too I suspect.)
> >
> > They are absolutely horrible on the brain when mixed with WARN_ON() interfaces,
> > which are the dominant runtime check interface in the kernel.
> >
> > Instead make it something like:
> >
> >   #define ct_state() (this_cpu_read(context_tracking.state))
> >
> >   #define CT_WARN_ON(cond) \
> >         WARN_ON(context_tracking_is_enabled() && (cond))
> >
> > and then the debug checks can be written as:
> >
> >         CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
> >
> > This is IMHO _far_ more readable than:
> >
> >         context_tracking_assert_state(CONTEXT_KERNEL);
> >
> > ok?
> >
> > (Assuming people will accept 'ct/CT' as an abbreviation for context tracking.)
> 
> Hmm, ok I guess.  The part I don't like is having ct_state() at all on
> non-context-tracking kernels -- it seems like it's asking for trouble.

Well:

 - if # CONFIG_CONTEXT_TRACKING is not se, then CT_WARN_ON() does nothing.

 - if CONFIG_CONTEXT_TRACKING=y, but !context_tracking_is_enabled(), then 
   CT_WARN_ON() will evaluate 'cond', but won't calculate it.

 - only if CONFIG_CONTEXT_TRACKING=y && context_tracking_is_enabled() should we 
   get as far as ct_state() evaluation.

so I'm not sure I see the problem you are seeing.

> We could make CT_WARN_ON not even evaluate its argument if 
> !CONFIG_CONTEXT_TRACKING, but then we still have ct_state() returning garbage if 
> !context_tracking_is_enabled().

My understanding is that if !context_tracking_is_enabled() then the compiler 
should not even try to evaluate the rest. This is why doing a NULL pointer check 
like this is safe:

  if (tsk && tsk->field) {
	...
  }

> The assert macro avoids all these problems despite being a bit ugly.

but writing good kernel code is all about not being ugly...

Thanks,

	Ingo